ArticlePDF Available

History and Development of the Scallop Fishery in Alaska

Authors:

Abstract

The weathervane scallop Patinopecten caurinus is a large, long- lived pectinid distributed from California to Alaska. A commercial dredge fishery from northern Southeast Alaska to the Bering Sea targets the species. The fishery developed rapidly in the late 1960s, declined sharply in the mid 1970s due to local depletion and availability of other fishing alternatives, and increased quickly in the late 1980s with improved stock conditions and prices. Fishery management evolved accordingly. Passive management regulations were replaced by active fishery management plans in the early 1990s in response to overcapitalization and resource conservation concerns. In recent years, fishery management plans have stabilized harvests at about 0.8 million pounds of shucked meats annually through guideline harvest levels and crab bycatch limits. An onbo ard observer program is a critical component of the fishery management process, providing important information on the biology, distribution, and relative abundance of Alaska's scallop stocks.
©Richard Carstensen 1994
Special Publication 14
A Workshop Examining
Potential Fishing Effects on
Population Dynamics and Benthic Community Structure
of Scallops with Emphasis on the Weathervane Scallop
Patinopecten caurinus in Alaskan Waters
June 10–12, 1999
Kodiak, Alaska
Alaska Department of
Fish and Game
and
University of Alaska
Fairbanks
September 2000
Alaska Department of Fish and Game
Division of Commercial Fisheries
P.O. Box 25526
Juneau, Alaska 99802-5526
Cite this publication as:
Alaska Department of Fish and Game and University of Alaska Fairbanks. 2000. A workshop examining
potential fishing effects on population dynamics and benthic community structure of scallops with
emphasis on the weathervane scallop Patinopecten caurinus in Alaskan waters. Alaska Department of
Fish and Game, Division of Commercial Fisheries, Special Publication 14, Juneau.
Cite individual papers within this publication as in the following example:
MacDonald, B. A. 2000. Potential impacts of increased particle concentrations on scallop feeding and
energetics. Pages 2026 in Alaska Department of Fish and Game and University of Alaska Fairbanks.
A workshop examining potential fishing effects on population dynamics and benthic community
structure of scallops with emphasis on the weathervane scallop Patinopecten caurinus in Alaskan
waters. Alaska Department of Fish and Game, Division of Commercial Fisheries, Special Publication
14, Juneau.
A Workshop Examining Potential Fishing Effects on
Population Dynamics and Benthic Community Structure of
Scallops with Emphasis on the Weathervane Scallop
Patinopecten caurinus in Alaskan Waters
June 10–12, 1999
Kodiak, Alaska
Alaska Department of Fish and Game
and
University of Alaska Fairbanks
Special Publication 14
Alaska Department of Fish and Game
Division of Commercial Fisheries
P.O. Box 25526
Juneau, Alaska 99802-5526
September 2000
iii
ABSTRACT
This document reviews the results of a workshop on scallop biology and the effects of
scallop dredging on benthic communities. The workshop was held in Kodiak, Alaska, during
10–12 June 1999. A review of the history of the Alaskan weathervane scallop fishery was
presented. Other speakers presented papers on scallop biology and fisheries in other cold-
water areas. Topics of the papers included physical and biological variables influencing
distribution, impacts of suspended particles on energetics, modeling approaches to identify
dredging impacts, effects of long-term dredging, benthic communities associated with
scallops, and the importance of protecting areas from fishing. Following the first day of
public presentations, a two-day workshop was convened to develop a viable study program
for examining the effects of dredging on the scallop’s life history, population dynamics, and
associated benthic community. The workshop results were intended to be applied to the
Alaskan fishery for weathervane scallops, but they are applicable to many scallop fisheries.
The working groups identified ten research topics for which information needs to be
gathered. Topics include the importance of spatial distribution on fertilization success, the
reproductive output of individuals, the importance of nursery areas, scallop behavior and how
it may be altered by dredging, factors that affect growth, fishery induced injury and
mortality, causes and rates of natural mortality, long-term factors affecting recruitment,
effects of scallop dredging on the benthos, and developing harvest strategies for scallops.
Also, the working groups recommended that a monitoring program be established that
included short- and long-term data gathering, and they identified methods and tools that
might be used for this task.
iv
ACKNOWLEDGMENTS
The Natural Resources Fund of the University of Alaska Fairbanks, Alaska Sea Grant, and
the Alaska Department of Fish and Game provided funding for this workshop. The workshop
coordinators were Kevin D. E. Stokesbury, Howard M. Feder, A. J. Paul, Doug Pengilly, and
Gordon H. Kruse. We thank the invited speakers: A. Brand, Marine Biological Station,
University of Liverpool, Port Erin, Isle of Man, U.K.; J. Grant, Department of Oceanogra-
phy, Dalhousie University, Halifax, Nova Scotia, Canada; B. MacDonald, Biology
Department, University of New Brunswick-Saint John, Canada; E. Kenchington, Department
of Fisheries and Oceans, Halifax, Nova Scotia, Canada; P. Dayton, Scripps Institution of
Oceanography, California; and session leaders T. Shirley and S. Jewett. We thank A. Tyler,
R. Dearborn, and N. Myers for their help and support. We also thank the University of
Alaska AnchorageKodiak Community College, Kodiak Island Borough, and the National
Marine Fisheries Service for providing meeting facilities.
v
TABLE OF CONTENTS
Abstract .................................................................................................................................... iii
Acknowledgments.....................................................................................................................iv
INTRODUCTION TO THE WORKSHOP......................................................................... 1
PART I: THE PRESENTATIONS
Introductory remarks by Kevin Stokesbury.................................................................... 4
History and Development of the Scallop Fishery in Alaska
Gordon H. Kruse, Jeffrey P. Barnhart, Gregg E. Rosenkranz, Fritz C. Funk and
Douglas Pengilly............................................................................................................. 6
Physical and Biological Variables Influencing the Spatial Distribution of the Giant
Scallop, Placopecten magellanicus
Kevin D. E. Stokesbury ............................................................................................... 13
Potential Impacts of Increased Particle Concentrations on Scallop Feeding and
Energetics
Bruce A. MacDonald................................................................................................... 20
Modelling Approaches to Dredging Impacts and Their Role in Scallop Population
Dynamics
Jon Grant ....................................................................................................................27
North Irish Sea Scallop Fisheries: Effects of 60 Years Dredging on Scallop
Populations and the Environment
A. R. Brand.................................................................................................................. 37
Benthic Faunal Species Associated with Scallop Grounds in the Bay of Fundy,
Canada
E. Kenchington............................................................................................................ 44
Problems in the Coastal Zone: A Generic Case for Marine Protected Areas
Paul K. Dayton............................................................................................................ 53
PART II: THE WORKSHOP.............................................................................................. 55
Research Needs for Preserving Scallop Stocks and Scallop Habitat........................... 55
Recommendations Identified by the Working Group .................................................. 64
LIST OF PARTICIPANTS.................................................................................................. 67
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
1
INTRODUCTION TO THE WORKSHOP
The weathervane scallop Patinopecten caurinus has been commercially fished in Alaska
since 1967. This fishery recently reached its highest value, due to increased landings and
price. Dredging, the primary harvesting method, may have a severe impact on sustainability
of scallop resources and the associated benthic community. We held a three-day international
workshop to review world experience on the effects of dredging on scallops, the present state
of the Alaskan fishery, and the community of organisms associated with scallop beds. A
working group of 23 scientists and agency biologists developed a study program to examine
the effects of dredging on scallop life history, population dynamics, and the associated
benthic community.
Little is known of weathervane scallop biology in Alaska, including spatial distribution and
abundance on large and fine scales, factors influencing fertilization success, the recruitment
process and how fishing may affect it, factors affecting growth and reproductive output of
individuals, mortality rates and causative factors, the timing of critical periods in the
scallop’s life history, genetic structure of the populations, and the habitat requirements and
associated species of the scallops. Dredging for scallops affects all of these processes in
unknown ways. Prudent management of the scallop resource requires resolution of these
uncertainties. The workshop focused on topics and scientific approaches that could be useful
in creating innovative management strategies that would allow sustainable harvests while
preserving the resource and minimizing damage to the habitat.
The University of Alaska Fairbanks (UAF) and the Alaska Department of Fish and Game
(ADF&G) invited the public to a day of lectures, on June 10, 1999, reviewing the present
state of the scallop fishery in Alaska and effects of dredging in other scallop fisheries
throughout the world. This was an opportunity for the public and fishing industry of Kodiak
to hear scientists from eastern Canada, the British Isles, and both coasts of the United States
to share their knowledge and experience. The extended abstracts from each of these lectures
are published in this document. The following two days were working sessions.
We structured these sessions on three biological and two temporal scales. The biological
scales and processes guidelines were:
Individual Population Community
growth spatial distribution competition
energetics abundance predation
physiology growth/age structure secession/recolonization
injury recruitment equilibrium shift
predation fertilization success trophic levels
competition genetics
Introduction
The participants were divided into two working groups:
Group I Group II
Name Affiliation Name Affiliation
J. Barnhart Alaska Department of
Fish and Game B. Bechtol Alaska Department of
Fish and Game
A. R. Brand University of Liverpool P. Dayton Scripps Institution of
Oceanography
J. Grant Dalhousie University H. Feder University of Alaska
Fairbanks
A. J. Paul University of Alaska
Fairbanks S. Jewett University of Alaska
Fairbanks
D. Pengilly Alaska Department of
Fish and Game E. Kenchington Department of Fisheries and
Oceans
G. Rosenkranz Alaska Department of
Fish and Game G. Kruse Alaska Department of
Fish and Game
T. Shirley University of Alaska
Fairbanks B. MacDonald University of New Brunswick
Saint John
K. Stokesbury University of Massachusetts
Dartmouth B. McConnaughey National Marine Fisheries
Service
B. Stone National Marine Fisheries
Service D. Woodby Alaska Department of
Fish and Game
C. Trowbridge Alaska Department of
Fish and Game
Focus questions of the working groups were:
What are the dominant species and community structures in undisturbed scallop habitats?
Does the community structure change after dredging at different intensities?
Does dredging influence recruitment, growth, and mortality of scallops left on the
grounds?
What are the main species interactions that influence interannual variation in recruitment
of scallops and benthic invertebrates? Which are the predators, the competitors, and the
symbionts?
What are the processes that account for the long-term changes in the assemblages of
species?
What are the main physical oceanographic events that would influence interannual
variation in recruitment of scallops and other benthic invertebrates associated with
scallop beds?
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
3
The temporal scales are short (approximately the life span of a scallop, 13 years) and long
term (decades).
Day 2: Focusing the general questions
Time Topic
0800–1000 Short term: Group I
Individual Group II
Population
1000–1015 Coffee
1015–1200 General meeting, presentation of two subgroups, and
discussion
1200–1300 Lunch
1300–1500 Long term: Group I
Population Group II
Community
1500–1515 Coffee
1515–1800 General meeting, presentation of two subgroups, and
discussion
Day 3: How to experimentally examine the questions identified during
Day 2
Time Topic
0800–1000 Short term: Group I
Individual Group II
Population
1000–1015 Coffee
1015–1200 General meeting, presentation of two subgroups, and
discussion
1200–1300 Lunch
1300–1500 Long term: Group I
Population Group II
Community
1500–1515 Coffee
1515–1800 General meeting, presentation of two subgroups, and
discussion
Presentations
PART I: THE PRESENTATIONS
Introductory remarks presented by Kevin Stokesbury on June 10, 1999.
I would like to welcome you all to the “Workshop Examining Potential Fishing Effects on
Population Dynamics and Benthic Community Structure of Scallops with Emphasis on the
Weathervane Scallop Patinopecten caurinus in Alaskan Waters.” I’m a marine biologist, and
a lot of my research deals with spatial distribution of fish and invertebrates. I’ve worked for
several years on the sea scallop and presently I’m working on the Georges Bank fishery. It
has been my pleasure to work with Howard Feder of UAF, and Gordon Kruse and Doug
Pengilly of the ADF&G to organize this workshop.
The weathervane scallop has been commercially fished in Alaskan coastal waters since 1967,
but relatively little is known of its life history. Further dredging, which is the primary
harvesting tool for this fishery, may have an impact on the benthic faunal community. The
driving idea of this workshop was to review world experience on the effects of dredging on
scallops, examine the present state of the Alaskan weathervane scallop fishery and the
community of organisms associated with scallop beds, and to develop a viable study program
examining the effects of scallop dredging on P. caurinus life history, population dynamics,
and associated benthic community. We wanted to find out what is being examined, how to
examine it, and how to avoid mistakes. To do this we wanted to bring together some of the
top researchers working on different aspects of scallop biology such as genetics, physiology,
ecology, fishery science, modeling, physical dynamics, and benthic community structure.
When Al Tyler and Howard Feder asked me to help them with this I couldn’t believe my
luck. I’m originally from the Maritime Provinces of Canada. I completed my Ph.D. at Laval
University with John Himmelman, working on a scallop ecology project funded by a
program called Ocean Production Enhancement Network (OPEN). OPEN focused on
scallops and Atlantic cod and provided a great deal of new information on these species. John
encouraged me to read as much about scallops as I could before writing my Ph.D. proposal.
One of the first articles I read was a chapter entitled “Scallop Ecology: Distributions and
Behaviour” by Andy Brand, in Sandra Shumway’s book on scallops. This chapter hooked me
on scallops and is an excellent overview of scallop ecology. Dr. Brand has an amazing
knowledge of scallops and has come from the Isle of Man to help us with this workshop.
There is a great deal of research on scallops. One of the best studies, which gave me insight
into scallop physiology and which I used as a meter stick to try to measure my work, was
Bruce MacDonald’s work on scallops in Newfoundland. Bruce is now at the University of
New Brunswick Saint John campus, and he and his students are working on a number of
different benthic marine invertebrates. Bruce was a principal investigator on OPEN and has
worked on the weathervane scallop in British Columbia.
Jon Grant was also a principal investigator on OPEN. His work includes carrying capacity of
aquaculture sites, sediment work in scallop beds and the effects of the benthic boundary
layer, oceanography, and modeling. His insights from an oceanographic perspective will be
key.
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
5
Before starting my Ph.D. I worked with the Department of Fisheries and Oceans, Canada
(DFO), in their invertebrate and marine plant section with Glyn Sharp. When Ellen
Kenchington began working at DFO Glyn described her research to me with very high
praise. As I read and heard more of Dr. Kenchington’s research I saw that the praise was well
deserved. Ellen’s work on scallop genetics and the difficult task of dealing with the scallop
fisheries of Nova Scotia will both be critical to our workshop.
Howard Feder suggested that we invite Paul Dayton. Although I had not previously met Dr.
Dayton, his research on marine ecosystems and community structure are world renown, and I
look forward to hearing his thoughts.
Further, from Alaska we have Tom Shirley, Steve Jewett, and A. J. Paul. During my two and
a half years at UAF I turned to Steve and Tom for advice and input on a number of different
topics. Both are top-notch marine scientists and have key local knowledge. I worked with
A. J. Paul on the SEA (Sound Ecosystem Enhancement) project; the way A. J. approaches a
scientific question is surgical. A. J.’s the most focused researcher I’ve ever met; he can cut a
question to the core.
So we have a dynamic group of researchers to help us with this workshop. We propose to
examine the present state of the Alaskan weathervane scallop fishery and to develop a viable
study program for examining the effects of dredging on the scallop’s life history, population
dynamics, and associated benthic community.
My work on Georges Bank has been both intense and political. Scallops are at the center of a
growing debate over the effects of mobile fishing on the marine ecosystem, fisheries
management, the use of natural resources, and the importance of a way of life. These
questions stretch beyond biology or science itself. However, at their core are three questions.
These three questions are:
1) What is the abundance of scallops, where are they located, and how does their life history
allow them to persist in that location?
2) What species will be collected as bycatch during fishing for these scallops?
3) What effect does the trawl have as it passes over the bottom, and how long does this
effect last?
History and Development of the
Scallop Fishery in Alaska
GORDON H. KRUSE
Alaska Department of Fish and Game, Division of Commercial Fisheries, P.O. Box 25526, Juneau, AK 99802-5526
Email: gordon_kruse@fishgame.state.ak.us
JEFFREY P. BARNHART
Alaska Department of Fish and Game, Division of Commercial Fisheries, 211 Mission Road, Kodiak, AK 99615-6399
Email: jeff_barnhart@fishgame.state.ak.us
GREGG E. ROSENKRANZ
Alaska Department of Fish and Game, Division of Commercial Fisheries, 211 Mission Road, Kodiak, AK 99615-6399
Email: gregg_rosenkranz@fishgame.state.ak.us
FRITZ C. FUNK
Alaska Department of Fish and Game, Division of Commercial Fisheries, P.O. Box 25526, Juneau, AK 99802-5526
Email: fritz_funk@fishgame.state.ak.us
DOUGLAS PENGILLY
Alaska Department of Fish and Game, Division of Commercial Fisheries, 211 Mission Road, Kodiak, AK 99615-6399
Email: doug_pengilly@fishgame.state.ak.us
ABSTRACT
The weathervane scallop Patinopecten caurinus is a large, long-lived pectinid distributed
from California to Alaska. A commercial dredge fishery from northern Southeast Alaska to
the Bering Sea targets the species. The fishery developed rapidly in the late 1960s, declined
sharply in the mid 1970s due to local depletion and availability of other fishing alternatives,
and increased quickly in the late 1980s with improved stock conditions and prices. Fishery
management evolved accordingly. Passive management regulations were replaced by active
fishery management plans in the early 1990s in response to overcapitalization and resource
conservation concerns. In recent years, fishery management plans have stabilized harvests at
about 0.8 million pounds of shucked meats annually through guideline harvest levels and
crab bycatch limits. An onboard observer program is a critical component of the fishery
management process, providing important information on the biology, distribution, and
relative abundance of Alaska’s scallop stocks.
INTRODUCTION
The purpose of this report is to provide a brief overview of biology, history, and management
of the weathervane scallop Patinopecten caurinus fishery in Alaska. The fishery is managed
with a precautionary approach given a lack of complete information on the species and its
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
7
productivity. Studies of scallop biology, abundance, productivity, and fishing effects are
critically needed to fill information voids so that fishery management can better strive toward
sustained optimal yields while minimizing adverse effects on other species and
habitats. A high level of observer coverage on a small fleet renders this a very tractable
fishery for research.
THE WEATHERVANE SCALLOP
Weathervane scallops are distributed from Point Reyes, California, to the Pribilof Islands,
Alaska (Foster 1991). They are found from intertidal waters to depths of 300 m (Foster
1991), but they tend to be most abundant between depths of 45–130 m on mud, clay, sand,
and gravel (Hennick 1973). Scallop beds tend to be elongated in the direction of mean
current flow. In Alaska, highest abundances of scallops exist off Yakutat, Kodiak Island, and
in the Bering Sea, with smaller aggregations occurring in Prince William Sound and off the
Alaska Peninsula and Aleutian Islands (Figure 1).
Most weathervane scallops mature at 76-mm shell height (SH) at about age 3 (Haynes and
Powell 1968, Hennick 1973). Funk (unpublished data) fitted Gompertz growth equations to
scallop data collected in the late 1960s and early 1970s by Hennick (1973). Scallops off
Yakutat grow much more slowly than scallops off Kodiak Island, and scallops off the west
side of Kodiak grow more slowly than those from the northeast side of Kodiak. The largest
recorded Alaskan specimen measured 250-mm SH and weighed 340 g (Hennick 1973).
Weathervane scallops are long-lived; the oldest Alaskan specimen was estimated to be 28
years old (Hennick 1973). Kruse (1994) estimated mortality rates for four areas in Alaska
using three different methods. Instantaneous natural mortality rates varied from 0.04 to 0.21
with a median of 0.13, corresponding to 12% annually.
FISHERY HISTORY
The Alaskan scallop fishery provides a classic example of fishery evolution through several
developmental stages: discovery and initiation of development, bandwagon growth, fallback,
and subsequent evolutionary development (Walters 1986). A fishery for weathervane
scallops in Alaska began in 1967 using paired New Bedford-style scallop dredges (Haynes
and Powell 1968). Within one year the fishery became fully developed when 19 vessels made
125 landings totaling 1.7 million pounds of shucked meats (Figure 2).1 Catches peaked in
1969 when 157 landings totaled 1.9 million pounds. Harvests off Kodiak and Yakutat
accounted for nearly all of the landings in the early years of the fishery. Whereas catches
from the early fishery were dominated by old scallops (7 years of age), landings shifted
toward younger ages (26 year olds) by the early 1970s (Hennick 1973). Landings declined
to 0.4 million pounds in 1975 as average landing per trip declined (Kaiser 1986). Less than
1 Meat recovery rate averages about 10% but varies between 9% and 11% depending on scallop size, season, and
area.
Presentations
three vessels participated in the fishery each year from 19761979. No vessels participated in
1978.
In the 1980s the weathervane scallop fishery received renewed interest due to increased
exvessel prices and recovering stock conditions. On an annual basis during the 1980s, an
average of nine vessels delivered 0.6 million pounds worth $2.15 million. Unlike the 1970s
when Kodiak and Yakutat accounted for 93% of the landings, during the 1980s 33% of the
landings were taken from Dutch Harbor and other areas such as Southeast Alaska, Cook
Inlet, Alaska Peninsula, and the Bering Sea.
In 1990 nine vessels made 144 deliveries that totaled 1.5 million pounds (Figure 2). By late
1992 landings exceeded 1.8 million pounds, the highest harvest since fishing on virgin
stocks. The fishing power of the fleet increased substantially in the 1990s. The number of
vessels increased from 4 in 1988 to 16 in 1993. Mean vessel length increased from 83 feet in
1981 to 110 feet in 1991, and mean crew size increased from 5.5 in 1984 to 12 in 1993. Some
vessels used automatic shucking machines. Concerns about resource conservation and fleet
overcapitalization led to new state (1994) and federal (1995) fishery management plans
(FMPs). As a result, statewide landings have averaged about 0.8 million pounds since 1996.
For more complete descriptions of the history of the Alaskan scallop fishery, see Kaiser
(1986), Kruse and Shirley (1994), and Shirley and Kruse (1995).
FISHERY MANAGEMENT
Prior to 1993 no FMP existed for scallops in Alaska. Rather, the fishery was managed by a
set of passive regulations, such as gear restrictions, closed areas to protect crabs, and fishing
seasons (ADF&G 1993, Kruse et al. 1992). Owing to increased landings, fishing power, and
resource conservation concerns in the early 1990s, the scallop fishery met the conditions of a
high-impact emerging fishery (5 AAC 39.210 in ADF&G 1993). Therefore, the Alaska
Department of Fish and Game (ADF&G) developed fishery management options (Kruse et
al. 1992), solicited public comment, and implemented an interim FMP and associated
regulations in 1993 (5 AAC 38.076 in ADF&G 1993). Later, a draft FMP (Kruse 1994) was
prepared to fully describe the rationale and strategy for scallop management and fishing
regulations. The Alaska Board of Fisheries (BOF) adopted a scallop FMP in March 1994,
and a current version appears in state regulations (5AAC 38.076 in ADF&G 1999).
In 1995 the National Marine Fisheries Service (NMFS) became involved in scallop fishery
management when the catcherprocessor vessel Mister Big fished in the Exclusive Economic
Zone (EEZ) without a State of Alaska permit. NMFS issued an emergency interim rule in
February 1995 to close federal waters to scallop fishing to prevent overfishing. In July 1995
the North Pacific Fishery Management Council (NPFMC) adopted a federal FMP to formally
close EEZ waters to scallop fishing. Since then, the federal FMP, including six amendments,
delegates most management to the State of Alaska.
Primary management objectives of the scallop FMP are to: (1) ensure long-term viability of
scallop populations, (2) minimize adverse effects of gear on habitat and other species,
(3) prosecute steady-paced fisheries that provide long-term socioeconomic benefits,
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
9
(4) maintain resource availability to subsistence users, and (5) conduct research to increase
knowledge for future management decisions. Key management measures to achieve these
objectives include establishment of nine registration areas, area closures to protect crab
habitat, a limited entry program to prevent overcapitalization, fishing seasons (July 1 through
February 15, except for August 15 to October 31 in Kamishak Bay), gear specifications (e.g.,
no more than two dredges of maximum size, 15 feet with 4-inch minimum ring size),
guideline harvest ranges for each area constrained by an overall cap of 1.24 million pounds
of shucked meats, crab bycatch limits set at 0.5% to 1% of the crab population, 100%
onboard observer coverage requirements, and efficiency controls (e.g., crew size limited to
12 and a ban on automatic shucking machines). Scallop regulatory proposals are reviewed
once every three years by the BOF and as needed by the NPFMC.
DATA COLLECTION AND FISHERY RESEARCH
ADF&G conducts a small research program on weathervane scallops to implement and
improve management of the fishery. The most important element is an onboard observer
program that was instituted in 1993. All scallop vessels, except those fishing in Kamishak
Bay, must carry an onboard observer at their own expense unless ADF&G waives this
requirement. The observer collects valuable information on fishing locations, bycatch and
scallop catch, size distributions, sex composition, reproductive condition, meat recovery, and
injury rates. Annual reports (e.g., Barnhart and Rosenkranz 1999) provide complete
descriptions and summaries of the observer data. A vessel operators’ logbook program
provides additional information on the fishery.
Collection of observer data has facilitated ongoing spatial analyses of scallop stock status and
productivity. The geographic distribution of scallop beds has been mapped, and depletion
estimators of abundance have been calculated for some beds. Ongoing aging studies are
examining reliability of growth rings as a measure of age. Size and age data are providing
valuable information for studies of recruitment. Preliminary analysis of biological reference
points from data collected in the late 1960s and early 1970s indicated target harvest rates of
12% to 14% and overfishing rates of 16% to 20% (F. Funk, unpublished data). Updated
analyses with contemporary observer data are planned. Other research needs include studies
of basic biology and life history, genetic stock structure, fishery-independent stock
assessments, population dynamics, gear catchability and selectivity, handling mortality, and
effects of scallop dredges on the sea floor.
REFERENCES
ADF&G (Alaska Department of Fish and Game). 1993. Commercial shellfish regulations,
1993 edition. Alaska Department of Fish and Game, Commercial Fisheries
Management and Development Division, Juneau.
ADF&G (Alaska Department of Fish and Game). 1999. 19992000 commercial shellfish
regulations. Alaska Department of Fish and Game, Division of Commercial Fisheries,
Juneau.
Presentations
Barnhart, J. P. and G. E. Rosenkranz. 1999. Summary and analysis of onboard observer
collected data from the 1997/1998 statewide commercial weathervane scallop fishery.
Alaska Department of Fish and Game, Division of Commercial Fisheries, Regional
Information Report 4K99-03, Kodiak.
Foster, N. R. 1991. Intertidal bivalves: a guide to the common marine bivalves of Alaska.
University of Alaska Press, Fairbanks.
Haynes, E. B. and G. C. Powell. 1968. A preliminary report on the Alaska sea scallopfishery
exploration, biology, and commercial processing. Alaska Department of Fish and Game,
Division of Commercial Fisheries, Informational Leaflet 125, Juneau.
Hennick, D. P. 1973. Sea scallop, Patinopecten caurinus, investigations in Alaska. Alaska
Department of Fish and Game, Division of Commercial Fisheries, Completion Report
5-3-R, Juneau.
Kaiser, R. J. 1986. Characteristics of the Pacific weathervane scallop (Pecten [Patinopecten]
caurinus, Gould 1850) fishery in Alaska, 19671981. Alaska Department of Fish and
Game, Division of Commercial Fisheries (Unpublished Report, catalog RUR-4K86-09),
Kodiak.
Kruse, G. H. 1994. Fishery management plan for commercial scallop fisheries in Alaska. Alaska
Department of Fish and Game, Commercial Fisheries Management and Development
Division, Draft Special Publication 5, Juneau.
Kruse, G. H., P. R. Larson and M. C. Murphy. 1992. Proposed interim management measures
for commercial scallop fisheries in Alaska. Alaska Department of Fish and Game,
Division of Commercial Fisheries, Regional Information Report 5J92-08, Juneau.
Kruse, G. H. and S. M. Shirley. 1994. The Alaskan scallop fishery and its management. Pages
170 –177 in N. F. Bourne, B. L. Bunting, and L. D. Townsend, editors. Proceedings of
the ninth international pectinid workshop. Canadian Technical Report of Fisheries and
Aquatic Sciences 1994.
Shirley, S. M. and G. H. Kruse. 1995. Development of the fishery for weathervane scallops,
Patinopecten caurinus (Gould, 1850), in Alaska. Journal of Shellfish Research 14: 71
78.
Walters, C. 1986. Adaptive management of renewable resources. MacMillan Publishing
Company, New York.
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
11
Figure 1. Locations of commercial scallop beds in Alaska.
#
A
l
a
s
k
a
P
e
n
i
n
s
u
l
a
#
K
o
d
i
a
k
I
s
l
a
n
d
A
l
e
u
t
i
a
n
I
s
l
a
n
d
s
B
e
r
i
n
g
S
e
a
#
P
r
i
n
c
e
W
i
l
l
i
a
m
S
o
u
n
d
#
Y
a
k
u
t
a
t
 

          
!" !#$%&$'$()*
+,* -!





        

    !
13
Physical and Biological Variables Influencing the Spatial
Distribution of the Giant Scallop Placopecten magellanicus
KEVIN D. E. STOKESBURY
Center for Marine Science and Technology, University of Massachusetts Dartmouth, 706 South Rodney French Blvd.
New Bedford, MA 02744 -1221
Email: kstokesbury@umassd.edu
ABSTRACT
Sea scallops Placopecten magellanicus aggregate on both a large (kilometer) and a small
(centimeter) scale. Large-scale aggregations are strongly associated with gravel substrates
while small-scale aggregations (clumps) are not. The short distance between scallops within
clumps, the high proportion of clumps with both sexes present, and an average of three
scallops per clump suggest high fertilization success within clumps. Comparisons of the
physical and biological conditions within scallop beds and in adjacent areas with low scallop
densities indicate that gravel substrate, low decapod predation, and presence of filamentous
flora and fauna are critical factors determining scallop aggregation location. Scallop move-
ment appears to be random, and scallops did not appear to migrate from unsuitable to suitable
habitats. However, scallops may move to form clumps resulting in increased fertilization
success. Recent surveys of Georges Bank closed areas indicated areas of high scallop
densities and highly aggregated distributions. These surveys demonstrate the positive effect
of allowing heavily fished scallop grounds a reprieve.
INTRODUCTION
Scallop distribution, on the scale of both kilometers and centimeters, is a result of a complex
mosaic of inter- and intraspecific interactions. We examined some of these interactions in
Port Daniel Bay, Quebec, Canada, during the summers of 1991 and 1992. We conducted
experiments examining the scallop’s spatial distribution on various scales (kilometer, meter,
and centimeter), the biological and physical factors influencing these distributions, and the
scallop’s ability migrate from unsuitable to suitable habitats. Conclusions from these
experiments provide information on scallop aggregations and are presented in chronological
order from recruitment to the adult reproduction.
Scallops were found on gravel or gravelsand substrates. However, not all gravel substrates
supported high densities of scallops. Temperature, salinity, and current direction and velocity
were similar inside and outside scallop aggregations in Port Daniel Bay.
Presentations
We found that filamentous flora and fauna distributions may influence where scallop spat
settles. High P. magellanicus spat settlement was not consistently associated with scallop
densities, but the filamentous organisms on which scallops settle were more abundant in the
scallop beds, possibly enhancing recruitment (Stokesbury and Himmelman 1995, Harvey et
al. 1993).
Following settlement, scallop survival is influenced by predation. Tethering experiments
indicated that the risk of predation was low within scallop beds compared to adjacent areas
(Stokesbury and Himmelman 1995). The intensity of asteroid predation was similar within
scallop beds and in surrounding areas with few scallops. The lobster Homarus americanus
was most abundant on bedrock and the rock crab Cancer irroratus on sand. Decapods
appeared to inflict considerable mortality on both small and large scallops. Highest
mortalities of small tethered scallops (35 45 mm in shell height) were on sandy bottoms
where rock crabs were most abundant, and shell remains indicated that most mortalities were
from predation by decapods. Mortality of large scallops (>70 mm) was correlated with
abundance of lobsters. A high proportion of the dead large scallops had broken shells,
similarly indicating decapod predation. It has been suggested that mollusks obtain a refuge
from decapod predation after they reach a specific shell height (Juanes 1992). Our field
research suggests that Placopecten magellanicus is not safe from predation by decapods
when it has attained a large size (Stokesbury and Himmelman 1995). This has significant
implications because decapod prey-size selection is an important component of molluscan
community structure.
The ratio of predation to predator density fluctuates between a linear and a nonlinear
relationship, depending on the scallop’s swimming ability. Movement did reduce predation
rates to 0% to 30% compared to mortalities (28% to 79%) estimated from tethered scallops;
however, a positive correlation between predator density and rate of scallop movement was
only found for one predator, C. irroratus. Therefore, scallops may move for reasons other
than to escape predation. Swimming ability is affected by the scallop’s size and by
environmental conditions such as photoperiod and temperature. Scallops increased their
movement in unsuitable habitats, dispersing randomly, and did not appear to migrate to
suitable habitats (Stokesbury and Himmelman 1996).
The scallop’s distribution may also be influenced by intraspecific interactions, for example,
the formation of clumps. I propose that the fertilization success of scallops is greatly
enhanced by the degree of clumping and that swimming in P. magellanicus may have
evolved so that individuals could form clumps, at the scale of centimeters, as well as to
escape from predators (Stokesbury and Himmelman 1993, 1996). The concentration of
gametes in the water column decreases with distance from the source. This suggests that
fertilization success may decrease exponentially with increased clump area, as gametes
would have greater distances to travel before they encounter gametes from the opposite sex.
Dredging has been found to change scallop distributions from contagious to random, and this
may decrease fertilization success (Langton and Robinson 1990). Thus distribution may
influence the number of spat initially in the water column and possibly the number available
for settlement, although many other variables are also associated with larval survival and
dispersion (Sinclair 1988, Tremblay et al. 1994). Finally, the contagious distribution of
  

   



   ! !        
!"!
   " #$$  %&
'($&)*  +,
!
"##-
+,    
 '. )"          
!             
/
!    
 !  !     ! 
* 00!

.  1         2 
* 00!% 34##










      




Presentations
We surveyed the scallop resource in Closed Area II of Georges Bank from 28 August to 5
October 1998 with the Fishermen’s Survival Fund of New Bedford, Massachusetts, the
NMFS at Woods Hole, and the Virginia Institute of Marine Science (VIMS). The objectives
of the Closed Area II survey were to determine the absolute abundance and spatial dis-
tribution of fishable scallops (number of scallopsm–2 >75-mm shell height). To do this we
calculated the efficiency of the New Bedford offshore scallop dredge from 39 depletion
experiments. We examined the size structure of scallops. We compared results from stations
in the Closed Area II to stations sampled adjacent to the south and west boundaries of the
Closed Area, where fishing is permitted.
METHODS
Six commercial fishing vessels were used, and the primary sampling gear was the New
Bedford offshore dredge which is 4.5-m wide, weighs approximately 1,870 kg, has three
tickler chains, a 20.3-mm diamond-mesh twine top, and a 4.5 x 0.8-m bag knit of 89-mm
steel rings.
To estimate the density of scallops in Closed Area II from the standard tow data we first
determined the area each tow sampled (about 0.0134 km2, 1.491-km tow distance x 9-m tow
width; about 0.0167 km2 including the set and retrieval distances). To calculate the number
of individuals sampled within a tow the mean shell height (in millimeters) per tow was
determined. The number of scallops within a basket is related to the scallops’ size by the
equation log y = 8.094 – 2.898 log x (r2 = 0.823, df = 525, P<0.001), where x is equal to mean
shell height per tow and y is equal to the number of individuals within a basket. The number
of individuals per basket was multiplied by the number of baskets collected per tow to
estimate the number of individuals collected per tow. This estimate agreed with the actual
number of scallops counted for shell height measurements and the number of recorded
baskets. Biomass was estimated by converting the number of scallops per area (x =
scallopsm–2) into meat weight (y, in grams) from the scallops’ shell height (log y = 4.416 +
2.819 log x; r2 = 0.94, df = 122, P<0.001).
The 39 depletion experiments provided an estimate of fishing efficiency. Depletion
experiments consisted of a series of 10-min tows repeatedly sampling the same area until the
catch was reduced to <25% of the initial tow. Depletion experiment locations were selected
based on preliminary scallop density (Figure 1). The Leslie model, which regresses catch per
unit effort against accumulative catch, was applied to these data. The slope of this regression
is an estimate of the gear’s catchability, and the x-intercept provides an estimate of the
population within the sample area. Each successive tow removes a fraction of the population,
and as a consequence the catch per effort declined proportionately. This decline in catch per
effort (baskets of scallops per tow) is called the catchability coefficient (q).
The assumption of a closed population, defined as those scallops within the average area
covered by the repetitive tows, is often violated in trawl depletion studies as it is difficult to
exactly repeat the tow track in open ocean conditions.
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
17
In order to estimate trawl efficiency from the depletion experiments, we first removed all
extreme geographical outliers where the vessel track markedly veered from the initial track
line. We determined the proportion of area that was sampled more than once using the
overlap factor 1A/a', where A is the total area fished one or more times and a' is the area
that would have been fished if the tows were adjacent to one another (a' = na; n = number of
tows). If there is no overlap 1A/a' = 0, and if there is complete overlap A/a' = 1/n. This
measure of overlap works well as a correction factor if the amount of overlap within the
experiment is high (1 A/a' > 0.4) but is unreliable at lower values.
RESULTS AND DISCUSSION
Two large areas in the north and south of Closed Area II supported high densities of scallops.
The northern aggregation covered approximately 857 km2 and contained 38.0% of the bio-
mass sampled. The most northerly quadrat contained 28.4% of the total biomass sampled.
The southern aggregation covered about 2,274 km2 and contained 49.6% of the biomass
sampled. These areas were separated by 2,915 km2 containing only 12.4% of the total
biomass. An average of 4.00 (SD = 8.6) baskets per tow was sampled within Closed Area II
while an average of 0.050 (SD = 0.74) baskets per tow was collected in the adjacent open area
(1,132 km2). There was eight times more biomass per area and six times more individuals per
square meter in Closed Area II than in adjacent open areas.
Twenty experiments had a high degree of overlap (minimum = 0.4, mean = 0.7, SD = 0.24).
These experiments estimated a mean trawl efficiency of 16.0% (SD = 6.49) when the cor-
rection factor for each experiment was applied.
The shell height (in millimeters) frequency for all scallops from Closed Area II was
symmetrical from 75 to 170 mm with a mean of 115.5 (SD = 18.91), and a second small
dome occurred below 75 mm.
Our estimates of the New Bedford offshore scallop dredge efficiency (16.0%, 95%
confidence limits = 2.84) agreed with independent estimates of offshore trawl efficiency using
scuba and video camera observations (15%, Bourne 1966; 15.4%, Caddy 1971; Stone and
Hurley 1987). Experimental modeling using maximum likelihood techniques to estimate
trawl efficiency suggests a high dredge efficiency (estimates vary with models with means
ranging from 23%, D. Cai, unpublished data, to 40%, P. Rago, personal communication,
NMFS, Woods Hole, Massachusetts). Given the critical role efficiency estimates play in
survey estimates of density, further experimentation in the field and with different analysis
techniques is essential.
Using a trawl efficiency of 16.0%, the relative density calculated from the standard tows,
the absolute biomass of fishable scallops within Closed Area II was estimated as about
37,340,000 kg (82 million lb; 60 million lb using set and retrieval distances). The absolute
estimate of about 37,000,000 kg meat weight for fishable scallops in Closed Area II is a
product of increased density (based on the scallops’ spatial distribution) and increased meat
weight with size. Scallops in the Closed Area II were larger than in the open area, and the
meat weight-to-shell length increases exponentially. For example, to obtain 1,000 kg of
Presentations
scallop meat in the open area, 58,343 scallops would be required. To obtain 58,343 scallops,
8.3 km2 would need to be scraped by the dredge. The mean age of these scallops is about 5
years old (Thouzeau et al. 1991), and they would produce 42.5 million eggs per female at the
time of harvest (McGarvey et al. 1992). To obtain 1,000 kg of scallop meat in Closed Area
II, 40,469 scallops would be required. To obtain 40,469 scallops, 0.93 km2 would need to be
scraped by the dredge. The mean age of these scallops is about 6.5 years old (Thouzeau et al.
1991), and they would produce 77.4 million eggs per female at the time of harvest
(McGarvey et al. 1992). Therefore, fishing in the Closed Area increases the mean age and
size in the catch by 1.5 years, doubling the meat weight per individual. The area scraped is
reduced substantially and so is the fishing mortality, but population fecundity is almost
doubled.
The positive effect of giving heavily fished areas of Georges Bank a reprieve from harvesting
is clearly demonstrated from this survey. The scallop population within Closed Area II is
mature and has a high density. Therefore a productive fishery is possible while at the same
time reducing fishing effort on Georges Bank. Further research is required to ensure nursery
areas are not disturbed and to identify possible areas for closure to allow presently heavily
fished areas a similar reprieve.
REFERENCES
Bourne, N. 1966. Relative fishing efficiency and selection of three types of scallop drags.
International Commission for the Northwest Atlantic Fisheries Research Bulletin 3:
15–25.
Caddy, J. F. 1971. Efficiency and selectivity of the Canadian offshore scallop dredge. ICES
Council Meeting Papers 1971/K25.
Harvey, M., E. Bourget and G. Miron. 1993. Settlement of Iceland scallop Chlamys islandica
spat in response to hydroids and filamentous red algaefield observations and
laboratory experiments. Marine Ecology Progress Series 99: 283–292.
Juanes, F. 1992. Why do decapod crustaceans prefer small-sized molluscan prey? Marine
Ecology Progress Series 87: 239–249.
Langton, R. W. and W. E. Robinson. 1990. Faunal associations on scallop grounds in the
western Gulf of Maine. Journal of Experimental Marine Biology and Ecology 144:
157–171.
McGarvey, R., F. M. Serchuk and I. A. McLaren. 1992. Statistics of reproduction and early
life history survival of the Georges Bank sea scallop (Placopecten magellanicus)
population. Journal of Northwest Atlantic Fishery Science 13: 83–99.
Sinclair, M. 1988. Marine populations: an essay on population regulation and speciation.
Washington Sea Grant Program, University of Washington Press. Seattle.
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
19
Stokesbury, K. D. E. and J. H. Himmelman. 1993. Spatial distribution of the Sea scallop
Placopecten magellanicus in unharvested beds in the Baie des Chaleurs, Québec.
Marine Ecology Progress Series 96: 159–168.
Stokesbury, K. D. E. and J. H. Himmelman. 1995. Biological and physical variables
associated with aggregations of the Sea scallop Placopecten magellanicus. Canadian
Journal of Fisheries and Aquatic Sciences 52: 743–753.
Stokesbury, K. D. E. and J. H. Himmelman. 1996. Experimental examination of movement
in the giant scallop Placopecten magellanicus. Marine Biology 124: 651660.
Stone, H. H. and G. V. Hurley. 1987. Scallop behaviour/fishing gear interactions.
Department of Fisheries and Oceans, Fisheries Development and Fishermen's
Services Division Project Report 123, Halifax.
Thouzeau, G., G. Robert and S. J. Smith. 1991. Spatial variability in distribution and growth
of juvenile and adult sea scallops Placopecten magellanicus (Gmelin) on eastern
Georges Bank (Northwest Atlantic). Marine Ecology Progress Series 74: 205–218.
Tremblay, M. J., J. W. Loder, F. E. Werner, C. E. Naimie, F. H. Page and M. M. Sinclair.
1994. Drift of sea scallop larvae Placopecten magellanicus on Georges Bank: a
model study of roles of mean advection, larval behavior and larval origin. Deep-Sea
Research Part II: Topical Studies in Oceanography 41(1): 749.
20
Potential Impacts of Increased Particle Concentrations on
Scallop Feeding and Energetics
BRUCE A. MACDONALD
Department of Biology and Centre for Coastal Studies and Aquaculture, University of New Brunswick
P.O. Box 5050, Saint John, New Brunswick, Canada E2L 4L5
Email: bmacdon@unbsj.ca
INTRODUCTION
There have been numerous studies looking at the effects of increasing particle concentration
on feeding in bivalves, including the sea scallop Placopecten magellanicus (e.g., MacDonald
and Thompson 1986, Cranford and Grant 1990, Cranford and Gordon 1992, MacDonald and
Ward 1994, Bacon et al. 1998, Cranford et al. 1998). Many of these studies have focused on
how the sea scallop will respond to changes in the concentration and quality of the suspended
food particles, which are highly variable and dependent on local biological and physical
conditions. Fluctuations in the concentration and quality of suspended particles results from
many natural processes including phytoplankton blooms, bioturbation, flocculation, erosion
of soils, and resuspension of sediments (Grant and Thorpe 1991). The concentration and
nutritional characteristics of the suspended particles may also be influenced on a large or
localized scale by man’s actions through the introduction of particles through construction
activity, dredging, offshore drilling, fishing activity, etc. Relatively high concentrations of
particles low in quality or organic content may be introduced into the water column thereby
“diluting” the naturally occurring nutritional particles and potentially impacting feeding and
production in local suspension-feeding bivalves (e.g., Widdows et al. 1979, Vahl 1980).
The objectives of this paper are to look at the possible effects that an increase in particle
concentration will have on short-term feeding activity and energy gain, and longer-term con-
sequences for growth and reproduction in scallops. To accomplish this I will use the sea
scallop from Newfoundland as the model species because a good database exists for studies
on physiological rates and production.
MATERIALS AND METHODS
The approach in this paper is to use published studies employing techniques of physiological
energetics in the field using natural seston with those in the laboratory environment using
artificial mixtures of particles to predict short-term energy gain (scope for growth, SFG)
under increasing particle concentrations. Predicted impacts of consistent increases in particle
concentration on SFG and the long-term consequences on growth and reproduction were
projected over a 13-year period.
Seston data for coastal Newfoundland waters were compiled from MacDonald and
Thompson (1986), MacDonald and Ward (1994), and unpublished data. Estimates of physio-
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
21
logical activity (e.g., clearance, ingestion, absorption efficiency, excretion, respiration, and
SFG) for scallops exposed to a controlled range of particle concentrations and organic quality
in the laboratory environment and then using natural seston were provided by Bacon et al.
(1998), MacDonald et al. (1998), and MacDonald and Thompson (1986), respectively.
Production (somatic growth and reproductive output) for various ages of scallops from
natural populations in Newfoundland was obtained from MacDonald and Thompson (1985).
RESULTS AND DISCUSSION
A reduction in clearance rate and production of pseudofaeces was observed in
P. magellanicus, as also reported for many other species, as a means to regulate ingestion
when particle concentration increased (Cranford and Gordon 1992, Bacon et al. 1998).
Despite the decline in clearance as concentration increased, ingestion of organic material still
increased because it is a product of clearance and organic concentration. In Newfoundland
coastal waters an increase in particle concentration from about 210 mgL1 resulted in some
“dilution” in the particulate organic matter (POM) of the seston from about 60% to
approximately 20% (Figure 1). While dilution of seston by particulate inorganic matter
(PIM) is quite common, Fegley et al. (1992) reported no such dilution by PIM in Great
Sound, New Jersey when concentrations increased three-fold over a tidal cycle. Obviously,
whether this “dilution effect” is observed will depend on the location studied and the nature
of the particles being added in suspension, and whether it is likely to have any impact on a
species will depend on its ability to “compensate” through changes in feeding activity and
selection.
Sea scallops have been shown to have the ability to preferentially reject poor-quality particles
when exposed to natural assemblages of particles in the field as well as mixtures of algal
cells and inorganics in the laboratory (MacDonald and Ward 1994, Bacon et al. 1998). The
authors of both of these papers reported that the efficiency with which P. magellanicus
selected particles decreased as POM decreased. A decrease in selection efficiency as POM
decreased was also recently reported for the green mussel Perna viridis (Wong and Cheung
1999).
Relationships of SFG for sea scallops exposed to 1, 3, 7, and 14 mgL–1 and POM levels of
25%, 50% and 80% derived from MacDonald et al.’s (1998) Figure 4 are presented in Figure
2. Note that a theoretical relationship for 5 mgL–1 was added between the 3 and 7 mgL–1
values for illustrative purposes. The calculation of these relationships represents the scallop’s
integrated response (e.g., pseudofaeces production, selection, ingestion and absorption rates,
excretion and respiration rates) when exposed to the various experimental conditions. SFG
decreases as POM and concentration of the seston decrease. If natural concentrations of
seston in Newfoundland increased according to the linear relationship in Figure 1, SFG
would follow the solid black line in Figure 2. On the other hand, if concentrations increased
according to the power relationship in Figure 1 SFG would follow the dashed black line in
Figure 2. While the shape of the SFG relationships may vary slightly, it is predicted that SFG
would continue to increase to a maximum of approximately 25 Jh–1g–1 as concentration
increased to 8 mgL–1. This would occur despite a drop in POM from about 60% to 20%;
 

 
               

     
  ! " ""        
    "        #" 
""  $"  "      $" 
"$ "   
  "             %
"$""   "  &  %" '()*+,   
$+)-.(
.(%" 
             "$""
( /  "',  
      %      
    % 
'+01201,"
34"5.( 3""$
  

             
           

 !"#$%&
             ! 
 "#'$     (       
 )*+ &    $,(

-     ./ 
  0 )*+, *  *
, 1./ 1
'23.4 51623.4 (*$
*    (      )*+$,  
(  ,       .4 $
     7   ""#$    
   1   (  6  8   
/  
0*,)*+(
 0 
   )*+(  3$  
1'23.4 51623.4 
 



 !
"!
     #$%
 &$''  
(       )*   
  )*  +    
#$%      
,

-! !-. */0 12 &334 ) 
# %   # %    
,
01*-1&35&(6&
7!)0!721!-8*&334
       #%  '
    ,      0 
1*-1&3596:(
 
$ #%!   #! )%  
   #! )%    ; # * / 
&34% )<      
&$
Potential Effects of Fishing on Scallops 1012 June 1999, Kodiak, Alaska
25
Cranford, P. J. and D. C. Gordon Jr. 1992. The influence of dilute clay suspensions on sea
scallop (Placopecten magellanicus) feeding activity and tissue growth. Netherlands
Journal of Sea Research 30: 107–120.
Cranford, P. J. and J. Grant. 1990. Particle clearance and absorption of phytoplankton and
detritus by the sea scallop Placopecten magellanicus (Gmelin). Journal of Experi-
mental Marine Biology and Ecology 137: 105–121.
Fegley, S. R., B. A. MacDonald and T. R. Jacobsen. 1992. Short-term variation in the
quantity and quality of seston available to benthic suspension-feeders. Estuarine,
Coastal and Shelf Science 34: 393412.
Grant, J. and B. Thorpe. 1991. Effects of suspended sediments on growth, respiration and
excretion of the soft-shell clam (Mya arenaria). Canadian Journal of Fisheries and
Aquatic Sciences 48: 1285–1292.
MacDonald, B. A., G. S. Bacon and J. E. Ward. 1998. Physiological responses of infaunal
(Mya arenaria) and epifaunal (Placopecten magellanicus) bivalves to variations in
the concentration and quality of suspended particles. II. Absorption efficiency and
scope for growth. Journal of Experimental Marine Biology and Ecology 219: 127
141.
MacDonald, B. A. and R. J. Thompson. 1985. Influence of temperature and food availability
on the ecological energetics of the giant scallop Placopecten magellanicus.
II. Reproductive output and total production. Marine Ecology Progress Series 25:
295–303.
MacDonald, B. A. and R. J. Thompson. 1986. Influence of temperature and food availability
on the ecological energetics of the giant scallop Placopecten magellanicus.
III. Physiological ecology, the gametogenic cycle and scope for growth. Marine
Biology 93: 37–48.
MacDonald, B. A. and J. E. Ward. 1994. Variation in food quality and particle selectivity in
the sea scallop Placopecten magellanicus (Mollusca: Bivalvia). Marine Ecology
Progress Series 108: 251–264.
Vahl, O. 1980. Seasonal variations in seston and the growth rate of the Iceland scallop,
Chlamys islandica (O. F. Müller) from Balsfjord, 70ÀN. Journal of Experimental
Marine Biology and Ecology 48: 195–204.
Widdows, J., P. Fieth and C. M. Worrall. 1979. Relationships between seston, available food
and feeding activity in the common mussel Mytilus edulis. Marine Biology 50: 195
207.
Presentations
Wong, W. H. and S. G. Cheung. 1999. Feeding behaviour of the green mussel, Perna viridis
(L.): responses to variation in seston quantity and quality. Journal of Experimental
Marine Biology and Ecology 236: 191–207.
27
Modelling Approaches to Dredging Impacts and Their Role in
Scallop Population Dynamics
JON GRANT
Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1
Email: jon.grant@dal.ca
INTRODUCTION
The dramatic decline in many coastal and shelf fisheries worldwide has focused attention on
fishing methods and their potential to impact benthic habitats. These concerns extend to the
mortality of target species that do not make up part of the catch but suffer indirect or
incidental mortality. There is similar concern about bycatch which may include commercially
valuable species. For example, in the context of invertebrate fisheries in Alaskan waters,
there are analogous issues that involve dredging for scallops Patinopecten caurinus and
bycatch of king crabs and Tanner crabs (Shirley and Kruse 1995). The potential habitat
alteration caused by mobile fishing gear has been the subject of a variety of recent studies
(Hall 1999). It is apparent that the impacts are dependent on sediment type, life history stage,
and functional group (infauna, epifauna, etc.; Collie et al. 1997, Thrush et al. 1998).
Although some effects, such as the disruption of colonial epifauna, are obvious, questions
remain about the implications of fishing practices for the population dynamics of the target
species. Despite the goal of optimizing fisheries yields, there are surprisingly few studies
which attempt to quantify how gear affects target species. This is a particularly relevant topic
for scallop fisheries since the gear is bottom directed (in contrast to some trawls), and the
target species is somewhat “delicate” compared to infaunal bivalves, which can burrow or
tightly close (Hall 1999). For the scallop example, there are more studies of dredge effects on
benthic communities than on scallop populations.
The present paper seeks to quantify some of the impacts of scallop dredging on scallop
populations and incorporate them into a yield per recruit model of a scallop cohort for the
weathervane scallop Patinopecten caurinus. Attempts have been made to utilize life history
parameters for this species based on Kruse et al. (2000), but additional information,
especially on bioenergetics, has been liberally borrowed from studies of the sea scallop
Placopecten magellanicus. These models will undoubtedly benefit from the firsthand
knowledge of scientists from the Alaska Department of Fish and Game.
The consequences of dredging for scallop populations may be defined in two broad
categories: habitat alteration and gear-induced damage and mortality. Habitat alteration
involves a broad variety of possible effects, including an array of linkages of scallops to the
benthic community, which are poorly known. Gear damage is of more direct consequence to
scallop mortality but is difficult to quantify as discussed below.
Presentations
HABITAT ALTERATION
There are at least two effects that may be examined in this regard, disruption of settlement
substrate for juvenile scallops and resuspension of sediments. The early life history of most
scallop species is poorly known, largely due to their small size at settlement. This is more so
for the spat of offshore species, which are difficult to sample on coarse bottoms. It appears
that byssal attachment is important to most pectinid spat and arborescent structures such as
eelgrass and colonial hydroids are known to be significant as settlement substrates.
Experiments with Icelandic scallops by Harvey et al. (1993) demonstrated that hydroids
collected orders of magnitude more spat than traditionally used monofilament collectors. The
implication is that removal of branched epifauna by fishing has a negative feedback to
scallop recruitment, but this relationship has not been sufficiently quantified to apply to
natural populations.
Sediment resuspension is perhaps the most obvious of bottom-gear habitat impacts and yet
the most poorly documented. Despite anecdotal reports of silting in scallops (e.g., Medcof
and Bourne 1964), there are no studies for dredges and only a single comprehensive study of
trawling impacts on sediment resuspension (Churchill 1989), including a model of
erosion/deposition. This work showed that although fishing-induced turbidity was at times
significant compared to natural resuspension, the absolute concentrations of suspended
sediment were relatively small (<1 mgL–1). Scallops are generally found in nonturbid waters
and the corresponding effects of turbidity on scallops are poorly known. Sea scallops can
benefit from organic matter present in resuspended sediment (Grant et al. 1997), although
they are sensitive to clay suspensions (Cranford and Gordon 1992). A large variety of bivalve
feeding studies demonstrate that excess turbidity decreases clearance rate and increases the
production of pseudofaeces. The rate at which clearance falls off with suspended sediment is
dependent on species as well as the nature of the suspension. Scallops are likely at the
sensitive end of the spectrum.
It should be noted that there are other sedimentary impacts and these are also sparsely
studied. Mayer et al. (1991) documented mixing of surface organic matter to sediment depth
and the return of reduced solutes to the surface. Yamamoto (1960; in Medcof and Bourne
1964) reports scallop mortality due to anaerobiosis caused by dredge disturbance of anoxic
sediment layers. Again, the implications of this sedimentary change for scallops and their
niche in the benthic community have barely been explored. As with suspended sediments,
scallops are sensitive to oxygen conditions.
The extent and duration of turbidity will depend on sediment type as well as the frequency
and depth of disturbance by gear. The most apparent effects will be over mud bottoms where
there is abundant fine material available for transport into the water column. On sand
bottoms, larger grains will be disturbed by the dredge, but their rapid settling rate ensures
that they contribute only briefly to local turbidity. In all cases, scallops ploughed into the
sediment by the dredge may have sediment forced into the valves (Medcof and Bourne
1964).
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
29
There may be other more subtle gear effects which influence scallop bioenergetics and can be
examined in the context of growth rate. Pectinids and a few other taxa are unique in their
swimming ability, a response noted frequently in observations of gear performance.
Swimming is energetically expensive, although Kleinman et al. (1996) found that adductor
condition was enhanced by swimming in juvenile sea scallops. As with feeding inhibition by
turbidity, increased swimming has the potential to add deficit to the scallop energy budget
and reduce growth, with resultant implications for fisheries yield.
MORTALITY
Underwater observations of scallop dredges demonstrate a variety of shell damage can occur
as a result of noncapture encounters with the gear (Shepard and Auster 1991). Again,
considering the potential importance of these encounters to the fishery, there are relatively
few directed studies in this area (Table 1). The basic approaches include seeding natural
scallop beds with marked (Gruffydd 1972) or unmarked scallops (Shepard and Auster 1991),
creating virgin beds (McLoughlin et al. 1991), or monitoring mortality in existing beds
(Caddy 1973, Naidu 1988). Depending on the methods used, these studies yield estimates of
indirect mortality (I) ranging from annual to daily time scales. The extrapolation of a single
dredge-contact event to the lifetime of a scallop is difficult since it is dependent on
subsequent damage and survival, as well as further fishing effort on the bed.
The general overview that can be gleaned from these studies is that: (1) damage is species-
dependent due to variation in swimming, byssal attachment and recessing, (2) damage is
substrate-dependent due to differing dredge behaviour on hard and soft bottoms; hard
Table 1. Incidental mortality studies for scallop populations
Study Location and species Gear Experiments Results
Gruffydd 1972 Isle of Man
Pecten maximus Manx 4-foot
dredge markrecapture 1056%; (M+I) = 0.10.8
Caddy 1973 New Brunswick
Placopecten magellanicus inshore and
offshore dredges submersible
observation 1317%; if annual,
I = 0.140.19
Naidu 1988 Newfoundland
Chlamys islandica inshore and
offshore dredges compare M fished
and unfished;
cluckers
I = 0.05
(max > 0.3)
McLoughlin
et al. 1991 Australia
Pecten fumatus Mud dredge,
4.8 m seed scallops in
“new” habitat 78%88% indirect
mortality
I = 1.52.1
Shepard and
Auster 1991 Maine
Placopecten magellanicus rock rake
(inshore dredge) seed scallops in
closed area 5–25% indirect mortality
I = 0.050.29
Presentations
substrates cause higher mortality due to lack of refugia (recesses, etc.), (3) size selectivity of
the dredge capture is poorly constrained and dependent on gear type and duration of fishing;
the exclusion of prerecruits cannot be guaranteed, and (4) incidental mortality (I) ranges from
values similar to natural mortality (M) to several times M and may be the chief source of
removal for some fisheries.
QUANTIFYING IMPACTS THROUGH MODELLING
A simple population model of a cohort’s production through its lifetime provides a means to
quantify some of the potential impacts arising from fishing. Yield per recruit modelling (see
Caddy 1989) uses an overall mortality term to follow cohort numbers through time
Nt = Nt–1 exp (Zt), (1)
where Nt and Nt–1 are population numbers at times t and t1, respectively, and Z is the total
mortality coefficient. For annual time periods where t = 1, the time term may be neglected. Z
may be partitioned as follows
Z = F + M + I, (2)
where F = fishing mortality, M = natural mortality and I = incidental mortality. The individual
contribution of these terms to temporal decline in cohort numbers proceeds via the catch
equation
Ca = Na exp (Z) (Fa/Za), (3)
where Ca = catch at age a and Fa is age-specific fishing mortality. Fa (partial recruitment) is
usually considered to be a “knife-edge” function in that recruitment into the fishery at young
ages (pre-commercial sizes) is low, increasing to 100% abruptly at the commercial size.
Fa/Za is the proportion of total mortality due to fishing and Z is age specific when Fa is
applied in Eq. 2. There are corresponding terms for other sources of mortality, similar to
Eq. 3, using (M/Z) and (I/Z), although the latter terms are not age specific. Ca multiplied by
age-specific biomass (e.g., von Bertalanffy growth) is the yield for a given age; the sum of
this yield over the cohort life span normalized to the number of recruits at age t0 is the yield
per recruit (YPR). YPR can be plotted versus F to determine a maximum yield as well as
other biological reference points (BRP). Because it includes fishing mortality, a single YPR
vs. F curve has important management information in it. Rather than focusing on BRPs, I
will compare the response of the curve to variation in bioenergetics and noncapture mortality
as detailed below.
While it is simple to produce the YPR model in a spreadsheet, use of a graphically based
simulation model readily allows interactive changes in model parameters, with immediate
graphical results. STELLA software (High Performance Systems Inc., www.hps-inc.com)
provides a highly interactive interface in which to examine the response of YPR to various
  

            
 !!"#$ !!"#

%&'()&* !"+#,
-   & . /000# 12,
 &       , 2
   -       3   
   4 &     2,  
&
5        &,     
2  06&
&  7/8 9 ,   *%, # % 
,2&, 06
  &    , ,
,    &
&,2  - & !!/# :
&     &  &   &  4
 !!/, !!;#<
= # 06& "6
&/!0#2
             4
    3&    2       
 , &   2      
/>6= #
= <




 

               
      
              
      
 !" !#   " $  
!     %   &"!!!' (     
))*&+#'
       !"& "',  
-!"". !.

       /     

0           & '
,
&'& 1.- !  "    
-!!'0    
! )             
    0         
2*3
 2          4566,
      , 4566, 7 8  
             
   3    
9
"
  

              
       
            !
"" !#"
$" !""%
  "
 
  "&

            
'
  "
 ""()
!"*
+,-"
Presentations
REFERENCES
Caddy, J. F. 1973. Underwater observations on tracks of dredges and trawls and some effects
of dredging on a scallop ground. Journal of the Fisheries Research Board of Canada
30: 173–180.
Caddy, J. F. 1989. A perspective on the population dynamics and assessment of scallop
fisheries, with special reference to the sea scallop Placopecten magellanicus. Pages
559–589 in J. F. Caddy, editor. Marine invertebrate fisheries: their assessment and
management. Wiley, New York.
Churchill, J. H. 1989. The effect of commercial trawling on sediment resuspension and
transport over the Middle Atlantic Bight continental shelf. Continental Shelf Research
9: 841–864.
Collie, J. S., G. A. Escanero and P. C. Valentine. 1997. Effects of bottom fishing on the
benthic megafauna of Georges Bank. Marine Ecology Progress Series 155: 159–172.
Costanza, R., D. Duplisea and U. Kautsky. 1998. Ecological modelling and economic
systems with STELLA. Ecological Modelling 110: 14.
Cranford, P. J. and D. C. Gordon. 1992. The influence of dilute clay suspensions on sea
scallop (Placopecten magellanicus) feeding activity and tissue growth. Netherlands
Journal of Sea Research 30: 107–120.
Curry, D. R. and G. D. Parry. 1996. Effects of scallop dredging on a soft sediment
community: a large scale experimental study. Marine Ecology Progress Series 134:
131–150.
Grant, J. and C. Bacher. 1998. Comparative models of mussel bioenergetics and their
validation at field culture sites. Journal of Experimental Marine Biology and Ecology
219: 2144.
Grant, J., C. W. Emerson and P. J. Cranford. 1997. Sediment resuspension rates, organic
matter quality, and food utilization by sea scallops. Journal of Marine Research 55:
965–994.
Gruffydd, Ll. D. 1972. Mortality of scallops on a Manx scallop bed due to fishing. Journal of
the Marine Biological Association of the United Kingdom 52: 449455.
Hall, S. J. 1999. The effects of fishing on marine ecosystems and communities. Blackwell
Science, Oxford.
Harvey, M., E. Bourget and G. Miron. 1993. Settlement of Iceland scallop Chlamys islandica
spat in response to hydroids and filamentous red algaefield observations and
laboratory experiments. Marine Ecology Progress Series 99: 283–292.
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
35
Haynes, E. B. and C. R. Hitz. 1971. Age and growth of the giant Pacific sea scallop,
Patinopecten caurinus, from the Strait of Georgia and outer Washington coast.
Journal of the Fisheries Research Board of Canada 28: 1335–1341.
Kleinman, S., B. G. Hatcher and R. E. Scheibling. 1996. Growth and content of energy
reserves in juvenile sea scallops, Placopecten magellanicus, as a function of
swimming frequency and water temperature in the laboratory. Marine Biology 124:
629–635.
Kruse, G. H., J. P. Barnhart, G. E. Rosenkranz, F. C. Funk and D. Pengilly. 2000. History
and development of the scallop fishery in Alaska. Pages 612 in Alaska Department
of Fish and Game and University of Alaska Fairbanks. A workshop examining
potential fishing effects on population dynamics and benthic community structure of
scallops with emphasis on the weathervane scallop (Patinopecten caurinus) in
Alaskan waters. Alaska Department of Fish and Game, Division of Commercial
Fisheries, Special Publication 14, Juneau.
Mayer, L. M., D. F. Schick, R. H. Findlay and D. L. Rice. 1991. Effects of commercial
dragging on sedimentary organic matter. Marine Environmental Research 31: 249
261.
McLoughlin, R. J., P. C. Young, R. B. Martin and J. Parslow. 1991. The Australian scallop
dredge: estimates of catching efficiency and associated indirect fishing mortality.
Fisheries Research 11: 1–24.
Medcof, J. C. and N. Bourne. 1964. Causes of mortality of the sea scallop Placopecten
magellanicus. Proceedings of the National Shellfisheries Association 53: 33–50.
Mohn, R. K. 1986. Generalizations and recent usages of yield per recruit analysis. Canadian
Special Publication of Fisheries and Aquatic Sciences 92: 318–325.
Naidu, K. S. 1988. Estimating mortality rates in the Iceland scallop, Chlamys islandica (O. F.
Müller). Journal of Shellfish Research 7: 61 71.
Shepard, A. N. and P. J. Auster. 1991. Incidental (non-dragging) damage to scallops caused
by dragging on rock and sand substrates. Pages 219–230 in S. Shumway and P. A.
Sandifer, editors. An international compendium of scallop biology and culture: a
tribute to James Mason. World Aquaculture Workshops 1. World Aquaculture
Society, Baton Rouge.
Shirley, S. M. and G. H. Kruse. 1995. Development of the fishery for weathervane scallops,
Patinopecten caurinus (Gould, 1850), in Alaska. Journal of Shellfish Research 14:
71–78.
Presentations
Thrush, S. F., J. E. Hewitt, V. J. Cummings, P. K. Dayton, M. Cryer, S. J. Turner, G. A.
Funnell, R. G. Budd, C. J. Milburn and M. R. Wilkinson. 1998. Disturbance of the
marine benthic habitat by commercial fishing: impacts at the scale of the fishery.
Ecological Applications 8: 866 879.





 !"#$%
 !"#!
    
        #&$   '$#
#$!  "   # 
 $   "            
  %#  &   $     '
" $$ "& $#  % '
   "  ( $
)   "# #     #  &
" $  " $$     (& '$ 
$$
!"$%#!$
 "$$  #  # 
$    !&   %*   # &$ +,-  
  '$##$+,-#&$
&      ./ $$  $         0    
  &$$     "#     &  
'$# #$  $#   " 1'$$  # &  $%$$ 
  23.    $    &$# $   &$$
 #   "  &   & !#  &  
$         #   
2431 /4           ! %$   '
"$ "  $$ $  
$ $  "'$ 
&+.11.-!"$ 
#  & $            
"$$
Presentations
DEVELOPMENT OF THE SCALLOP AND QUEEN FISHERIES
The scallop fishery started in 1937 with a few small boats fishing grounds close inshore off
the west coast of the Isle of Man. After the war the fishery developed rapidly, more and
larger boats joined in each year and new grounds further offshore were exploited. In the early
years the individual dredges were comparatively large (3.56.0-foot wide) with a fixed tooth-
bar, and these were grouped together in “gangs” on a heavy steel towing bar. “Newhaven”
spring-tooth dredges were introduced in 1972 (Mason 1983) and rapidly replaced fixed tooth-
bar dredges as the normal gear. Then in the early 1980s, individual dredge size was reduced
and 2.0- or 2.5-foot spring-tooth dredges have since become more-or-less universal through-
out the British Isles. The number of dredges varies with the power of the boat and 4–12 per
side is normal, but very big vessels sometimes use larger spreads. These gear developments
enabled the boats to efficiently exploit rougher areas of seabed. The start of the queen fishery
in 1969 was also a major influence on the exploitation of new scallop grounds. The two
species coexist in many areas, so scallops have subsequently been fished in many areas
where they occur in densities that would not be viable were it not for the additional queen
bycatch. Since the mid 1980s scallop fishing has been taking place on a number of more-or-
less distinct fishing grounds all around the Isle of Man (Figure 1a), while queen fishing
grounds are mostly to the north, east, and south of the island (Figure 1b).
In the early years of the queen fishery various types of dredges without teeth were tried but
Newhaven spring-tooth dredges rigged with shorter, more closely set teeth and smaller ring-
diameter bellies were found to be most efficient and have now been generally adopted.
Because of the large amount of bottom debris retained by the small-mesh bellies, boats
dredging for queens use mechanical riddles on deck to sort the catch. On some grounds
bottom trawls are also very efficient at catching queens in the summer months, when the
queens actively swim to avoid capture, and some boats trawl for queens for a few months
each year (Brand et al. 1991).
Since 1943 there has been a summer closed season for scallop fishing (JuneOctober
inclusive) and a minimum legal landing size (110-mm shell length) has been enforced. There
are no specific regulations restricting queen fishing, but catches with a high proportion of
queens below 55-mm shell height are not usually commercially acceptable. For all boats,
there are certain boat and gear size restrictions within the Isle of Man 3-mile territorial zone.
The various scallop and queen fishing grounds around the Isle of Man therefore differ in the
historical duration, intensity, and annual pattern of exploitation, as well as in environmental
factors such as depth and bottom type. This provides an unusual opportunity to study the
long-term effects of scallop dredging on both the scallop populations and the environment.
EFFECTS OF FISHING ON SCALLOP POPULATIONS
For many fishing grounds a discontinuous series of catch per unit effort data (CPUE,
expressed here as numbers of scallops per metre dredge width per hour’s fishing) are
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
39
available and provide estimates of scallop abundance covering the historical duration of
exploitation. Early estimates of scallop abundance on many grounds show initial CPUEs of
>150 scallopsm–1h–1. These fell rapidly on all grounds within a few years after fishing
commenced. Since 1981, when detailed CPUE data started to be collected on an unusually
small spatial scale (5 x 5-nautical mile grid), scallop abundances have been uniformly low at
around 20 scallopsm1.h1 on all grounds, but with a general downward trend. CPUE can
vary considerably from year to year (usually within the range of 10–40 scallopsm–1h–1),
reflecting differences in both recruitment and exploitation. However, while these variations
are of commercial significance, they represent small changes in scallop abundance in relation
to the huge changes that followed exploitation of the virgin fisheries.
Figure 1. Scallop Pecten maximus and queen Aequipecten opercularis fishing grounds
currently fished around
the Isle of Man. Major fishing grounds are bounded by solid
lines; a dotted outline indicates areas where scallops occur and are occasionally
fished. All boundaries are approximate and many fishing grounds are contiguous.
a) Scallop fishing grounds: 1The Targets, 2Kirkmichael Bank, 3
Peel Head,
4–Bradda Inshore, 5Offshore Bradda/West Calf, 6The Chickens, 7
Port St.
Mary, 8Port St. Mary Offshore, 9H/I Sector and Offshore South, 10
Southeast
Douglas, 11East Douglas, 12Laxey, 13Maughold Head, 14Ramsey, 15
Point
of Ayre. b) Queen fishing grounds: 1The Targets, 2H/I Sector, 3
Southeast
Douglas, 4East Douglas, 5Laxey Bay, 6Maughold Head, 7
Ramsey Bay,
8–Point of Ayre.
Presentations
Typical estimates of scallop densities for the Bradda Inshore fishing ground show a long-
term decline from 920 scallops100 m
–2 in the 1950s and 1960s to <7 scallops100 m
–2
between 1981 and 1984, <4 scallops100 m
–2 in 19861990, and <3 scallops100 m
–2 in all
surveys since then. Low densities of <3 scallops100 m
–2 are now typical of most of the
inshore fishing areas around the Isle of Man and many of the offshore grounds. For the most
heavily exploited fishing grounds, where young scallops dominate the populations, there are
large seasonal fluctuations in density. Preseason (October) densities on all the heavily fished
grounds are now around 3 scallops100 m
–2, depending on the strength of summer recruit-
ment, but fall to about 1.5 scallops100 m–2 by the end of the fishing season. This latter value
probably approximates the density that the fishermen currently consider uneconomic; when
density falls to this level they move off to fish elsewhere.
For the scallop populations on most fishing grounds, long series of age composition data are
available. These show the progressive reliance of the fishery on young scallops as the older
age classes become depleted. This characteristic pattern of change is shown as a series of
cumulative age frequency curves (Figure 2a) for the Bradda Inshore fishing ground, the
longest and most intensively exploited ground in the north Irish Sea. For the last 1520 years
up to 70% of the catch has been 4 years old or less, and mainly below the minimum legal
landing size (110 mm). Similar patterns of change have occurred on all the other fishing
grounds and scallops of 6 years or older are only present in any quantity on some of the less
heavily fished offshore grounds (Figure 2b). Continuous heavy exploitation has therefore
lead to stocks dominated by young scallops, high rates of discarding, and fisheries that are
highly dependent on the strength of annual recruitment. Fortunately, recruitment in the north
Irish Sea has been remarkably consistent from year to year, as indicated by the smooth form
of the cumulative frequency curves. Where annual recruitment is less regular, heavy
exploitation typically leads to “boom and bust” scallop fisheries (Young and Martin 1989).
ENVIRONMENTAL EFFECTS OF SCALLOP DREDGING
For the last 6 years a large research programme has studied the environmental impact of
scallop dredging around the Isle of Man, including both short-term (hours, days) and long-
term (weeks, months) effects (Hill et al. 1997). Three aspects of this work will be considered
briefly here: studies of scallop and queen dredge bycatch, comparisons of present benthic
communities with a historical dataset, and studies carried out in a closed area.
Detailed studies of the bycatch of scallop and queen dredges have been made on 15 fishing
grounds that differ in environmental conditions (depth, bottom substrate, etc.) as well as the
historical duration and annual intensity of fishing (both of which are known with some
accuracy). Visual assessments of damage, supported by laboratory survival experiments,
show that some invertebrate groups are more vulnerable to capture than others. Brittle or
fragile animals such as the urchins Spatangus purpuratus and Echinus esculentus, the
brittlestar Ophiocomina nigra, starfish Anseropoda placenta, and edible crab Cancer pagurus
all suffer badly in the dredges, while animals with more robust bodies, like the cushion star
Porania pulvillus, or those with thick shells, such as the gastropod Colus gracilis and hermit
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
41
crabs, have a much lower sensitivity. Queen dredges, by virtue of their more closely set teeth
and smaller belly rings, catch and kill a greater number of individuals, species, and biomass
of bycatch animals than scallop dredges. Both univariate and multivariate analyses have
shown statistically significant relationships between the bycatch assemblage structure and
long-term (15-year mean) fishing effort, while associations with other variables such as depth
and substrate type are not so strong (Bradshaw et al. in press). Prolonged commercial scallop
Figure 2. Cumulative age frequency curves for scallops from the Bradda Inshore ground in
various years during the development of the fishery (a) and for various fishing
grounds in 1995 (b).
Presentations
dredging has therefore permanently affected community composition.
Detailed studies of the benthic communities around the south of the Isle of Man were carried
out during the period 19351955, before the scallop dredge fishery started on some grounds
(Jones 1940, 1951). Since the original notebooks for these studies are still available it has
been possible to reanalyze Jones’ original data using modern techniques and to resample
some of his sites using similar gear. The differences in benthic community composition
between the historical and current samples are remarkable and show comparatively little
overlap (Hill et al. 1999). It is not possible, of course, to establish cause and effect since
factors other than fishing may have changed over the last 40 years. However, such studies
allow us at least to predict the likely effects of fishing on benthic communities and to
determine what species or groups may be sensitive to fishing disturbance.
More precisely controlled studies of the effects of scallop dredging have been carried out in a
closed area. This fishing exclusion zone of nearly 2 km2 has been closed to commercial
fishing with towed gear since 1989; prior to that it was heavily dredged for 50 years, and the
surrounding area continues to be one of the most intensively fished grounds in the British
Isles (Brand and Prudden 1997). This is a most valuable experimental facility for it has
enabled us to study the recovery of benthic communities after the cessation of fishing, as well
as to carry out comparative studies of dredged and undredged plots, inside and outside the
closed area.
In the natural recovery of the closed area many epifaunal species have increased in
abundance including Pecten maximus, Luidia ciliaris, hermit crabs, spider crabs, brittlestars
and upright sessile species such as Pectinaria koreni, Cellaria spp. and Polycarpa spp.
Conversely, the predatory starfish Asterias rubens has decreased in abundance.
Benthic communities have been compared from experimentally dredged and undisturbed
plots within the closed area, and with adjacent plots subjected to high levels of commercial
dredging outside the closed area. Multivariate cluster analysis has shown that the benthic
communities in the closed area (not dredged for 5 years) were initially more diverse than in
the fished areas outside. However, since experimental dredging began in the closed area, the
infaunal communities of the dredged plots have become more similar to those of the
commercially dredged grounds than to the undisturbed closed area plots. This is some of the
strongest evidence in this study showing the effects of dredge disturbance on benthic
community structure.
In conclusion, these studies have shown that scallop dredging does affect the benthos in
many ways. In general, there is a loss of biodiversity, with more polychaetes and fewer
molluscs and other long-lived species, fewer fragile animals like echinoids and certain
starfish, and at least a short-term loss of erect filter feeders such as hydroids and bryozoans
(which may be important settlement sites for scallop spat). Finally, we have no evidence to
support the popular hypothesis that populations of benthic scavengers benefit greatly from
bycatch carrion. This is probably because the annual pattern of scallop dredging does not
provide a regular food supply for benthic scavengers with limited mobility.
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
43
REFERENCES
Bradshaw, C., L. O. Veale, A. S. Hill and A. R. Brand. In press. The effect of fishing on
gravelly seabed communities. In M. J. Kaiser and S. J. de Groot, editors. Effects of
fishing on non-target species and habitats. Fishing News Books, Oxford.
Brand, A. R., E. H. Allison and E. J. Murphy. 1991. North Irish Sea scallop fisheries: a
review of changes. Pages 204218 in S. E. Shumway and P.A. Sandifer, editors. An
international compendium of scallop biology and culture: a tribute to James Mason.
World Aquaculture Workshops 1, World Aquaculture Society, Baton Rouge.
Brand, A. R. and K. L. Prudden. 1997. The Isle of Man scallop and queen fisheries: past,
present and future. Report to Isle of Man Department of Agriculture, Fisheries and
Forestry by Port Erin Marine Laboratory, University of Liverpool.
Hill, A. S., A. R. Brand, L. O. Veale and S. J. Hawkins. 1997. Assessment of the effects of
scallop dredging on benthic communities. Final Report to Ministry of Agriculture,
Fisheries and Food, U.K. (Contract CSA 2332).
Hill, A. S., L. O. Veale, D. Pennington, S. G. Whyte, A. R. Brand and R. G. Hartnoll. 1999.
Changes in Irish Sea benthos: possible effects of forty years of dredging. Estuarine,
Coastal and Shelf Science 48: 739–750.
Jones, N. S. 1940. The distribution of the marine fauna and bottom deposits off Port Erin.
Proceedings of the Liverpool Biological Society 53: 1–34.
Jones, N. S. 1951. The bottom fauna off the south of the Isle of Man. Journal of Animal
Ecology 20: 132–144.
Mason, J. 1983. Scallop and queen fishing in the British Isles. Fishing News Books, London.
Young, P. and R. B. Martin. 1989. The scallop fisheries of Australia and their management.
Reviews in Aquatic Sciences 1: 615– 638.
44
Benthic Faunal Species Associated with Scallop Grounds in the
Bay of Fundy, Canada
E. KENCHINGTON
Department of Fisheries and Oceans, Bedford Institute of Oceanography
Dartmouth, Nova Scotia, Canada B2Y 4A1
Email: KenchingtonE@mar.dfo-mpo.gc.ca
INTRODUCTION
Human activities are affecting marine benthic communities that have yet to be fully
understood or examined in a pre-disturbance state. As fishing activity is intensive, fairly
continuous, and often disturbs a given area more than once in a season, there is likely very
little of the sea floor in existence that is representative of an “old growth” benthic community
(Auster et al. 1996). Collecting information on species present in scallop drags will identify
those affected directly by the fishing process. This information is important in understanding
the benthic ecosystem and in making management decisions that support sustainability of
marine resources.
The Lower Bay of Fundy is one of the prime fishing areas for the sea scallop Placopecten
magellanicus and home of the “Digby scallop.” It is also an area that supports a wide variety
of bottom types within a relatively small area. Caddy (1973) examined both dragging and
trawling on scallop grounds in the Bay of Fundy and observed effects on both surficial
geology and on the scallop populations. The geological component of the benthos in the
Lower Bay of Fundy is comprised mainly of Scotian Shelf Drift, which as described by Fader
et al. (1977) is made up of a poorly sorted mixture of sand, clay, pebbles, boulders, and
cobbles. Recent work with side-scan sonar, multibeam imaging and seismic techniques has
provided insight into the surface topography and stratigraphy of the Bay of Fundy (G. Fader,
Atlantic GeoScience Center, BIO, Dartmouth, Canada). Features identified in these images
include mussel reefs (referred to as bioherms), shell deposits, sand dunes, ripples of various
sizes, fault lines, iceberg troughs, scouring and erosion fields, and inevitably, trawling
impacts. The resolution is such that the characteristic paired otter-trawl door tracks can easily
be distinguished from those of the multiple “buckets” of the scallop gear. These tracks are
locally persistent for over 12 months. Their contribution to the dynamics of the surficial
geology within the bay are as yet unknown and is likely to depend upon whether the area
impacted is one of erosion or deposition.
Previous studies of the associated species inhabiting scallop grounds in the northwest
Atlantic include work in the Lower Bay of Fundy (Caddy 1970, Caddy and Carter 1984), the
Gulf of Maine (Langton and Uzmann 1989, Langton and Robinson 1990) and Georges Bank
(Thouzeau et al. 1991). Because of the unique physical characteristics of the Bay of Fundy, it
is not yet clear whether or not faunal associations and their response to anthropogenic
disturbance are the same as in other areas in the FundyMaineGeorges larger ecosystem
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
45
(Percy et al. 1996). Communities in the sublittoral benthos of the Lower Bay of Fundy have
been studied by Logan and Noble (1971), MacKay (1975), Wildish and Peer (1983), Wildish
(1984), and Logan et al. (1986).
Benthic species associated with commercial scallop grounds in the Lower Bay of Fundy were
inventoried by sorting faunal bycatch during the 1997 Department of Fisheries and Oceans
inshore scallop population surveys. Details of this study are reported in Fuller et al. (1998).
All tows were made with four-gang gear consisting of drags with an inside width of 76 cm.
Known scallop grounds off of Digby, Yarmouth/Brier Island, Nova Scotia, and Grand
Manan, New Brunswick were included in the survey. These areas differ in geography and
tidal and current influence with respect to their proximity to the entrance of the Bay of
Fundy. Exact tow locations can be found in Fuller et al. (1998). Depth ranged from 12 to
152 m (mean = 76 m, standard deviation = 31 m). The contents of 234 tows were assessed and
species were recorded on a presence/absence basis providing data both on the frequency of
occurrence and distribution. Epifaunal species occurring on the sea scallop Placopecten
magellanicus were recorded, as well as the level of epifaunal encrustation. A total of 261
taxa were identified to at least family level and often to species level. Thirteen phyla and 131
families were present.
MAJOR GROUPS OF ORGANISMS FOUND ASSOCIATED WITH SCALLOP
BEDS
Porifera: A total of 23 species of sponges (Porifera) were identified. Sponges are integral
members of the benthic community and provide habitat for hundreds of other species
(Klitgaard 1995). Observations from this study and by Caddy (1970) indicate that the area
above Digby Gut has high densities of branching sponges. Species included in this
assemblage are Haliclona oculata, H. urceolus, Isodictya deichmannae, I. palmata, and
Esperiopsis normani. Sponges are suspension feeders and thrive in areas of high current
velocity. In some areas both diversity and abundance of sponges was high. Two taxa, Iophon
sp. and Pseudosuberites sulphureus, were observed to be growing almost exclusively on the
brachiopod Terebratulina septentrionalis.
Coelenterata: Hydroids comprised a significant part of the benthic community, especially
on shell-debris substrates in the area above Digby Gut. Like the sponges, hydroids are
frequently overlooked in studies of the benthos. Hydrallmania falcata, Sertularia pumila, and
Sertularella polyzonias were often observed in dense quantities and were also ubiquitous in
distribution. Presence of these species provides a niche for other organisms and high
abundances of Ophiopholis aculeata, Nymphon spp., and Hyas spp. were found in association
with dense hydroid cover. Anemones prefer hard substrates and were generally present in all
areas, with the exception of very muddy or sandy tows. They were not the most frequently
encountered faunal unit inhabiting the benthic community of scallop grounds off Digby as
reported by Caddy and Carter in 1984. Soft corals were also collected.
Bryozoa: The most conspicuous bryozoan was the leafy species Flustra foliacea, commonly
referred to as lemon weed, which was recorded very frequently and in high densities in the
Digby area, but rarely encountered in the other study sites. This species appears to be
Presentations
46
increasing in distribution and fishers acknowledge that trawling impacts its range. The
increase in range is associated with a reduction of effort in the area. Eucratea loricata was
described to be the most abundant branching bryozoan in the Digby area by Caddy (1970).
Although this species was often present, it was not abundant in any particular area. The
effects of this apparent change in epifaunal distribution on the rest of the benthic community
have yet to be studied and may reflect fishing activities or cycles in abundance. Encrusting
bryozoans were generally always present in areas with rock and shell debris as substrates.
Evidence from Collie et al. (1996) indicates that these organisms were some of the few that
were not affected by trawling activity.
Brachiopoda: The brachiopod community in the Lower Bay of Fundy has been extensively
studied by Logan and Noble (1971), Logan et al. (1975), Noble et al. (1976), and Logan et al.
(1986). Terebratulina septentrionalis is the only species occurring in the area. Very dense
populations of T. septentrionalis tended to occur in deeper water, and this species was
present most frequently in tows off of Yarmouth.
Polychaeta: Drag sampling does not adequately sample infaunal species, and those collected
tend to be filtered out as the drag is brought to the surface. Forty-two species of polychaetes
were identified at least to genus. The tube building worms Spirorbis borealis and Filograna
implexa were the most frequently observed polychaetes in this study. Both species build
calcareous tubes and are found encrusting on both shells and rocks. Filograna implexa tends
to grow in dense mats inside empty Modiolus shells when they are present (this study, Collie
et al. 1996). Potamilla reniformis and Thelepus cinncinatus were associated with
Placopecten magellanicus in all areas. Spirorbis spirorbis was observed to be associated with
hydroids, and abundance was directly proportional to hydroid abundance.
Molluscs: The most frequently observed mollusc was the sea scallop Placopecten
magellanicus. Population and demographic survey results for the scallop grounds discussed
in this report are available (Kenchington and Smith 1997, Kenchington et al. 1997). Fifty
other species including mussels, chitons, moonsnails, and whelks were also present. The
waved whelk Buccinum undatum and the wrinkle whelk Neptunea lyrata decemcostata were
by far the most abundant and frequently occurring gastropods in all study sites. In all study
sites gastropod diversity was highest in sand/gravel substrates.
Cephalopoda: The most common cephalopod was the small spoonarm octopus Bathypolypus
arcticus found in deep water areas in both Digby and Grand Manan. Recent research efforts
have investigated the biology of this species in this area (Wood et al. 1996, 1998).
Crustacea: Although the sampling method was far from optimal for assessing the amphipod
component of the benthos, several species were collected and identified in this study; fewer
isopods were recorded. The most commonly occurring species were Leptocherius pingius
and Erichthonius spp. All belong to the Aoridae family, which are tube and nest dwelling
(Bousfield 1960) and therefore tend to reside in the sediments as opposed to inhabiting the
hyperbenthos. This makes it more likely to catch such specimens in drag and grab samples.
The corophid Unciola irroratus was also collected often, and is tube dwelling as well.
Several species of Caprellidae were collected; Aeginina longicornis and Caprella
Potential Effects of Fishing on Scallops 10–12 June 1999, Kodiak, Alaska
47
septentrionalis were most commonly observed. High densities of both species were observed
in association with branching sponges Halichondria panacea hydroids, and in the interstices
of sandy substrate. Caprellids tended to be attached to emergent epifaunal species like
sponges and hydroids. Both Cancer borealis and C. irroratus were recorded from all sites
and all substrate types, and presence did not appear to be affected by depth. Hyas araneus
and H. coarctatus were most abundant in association with hydroid and sponge epifaunal
communities. Specimens often had sponges, hydroids, and barnacles colonizing the carapace.
The hermit crabs Pagurus acadianus and P. pubescens were ubiquitous and particularly
abundant in the area of highest gastropod density (an abundance of shells for houses).
Lobsters were also captured.
Echinodermata: The echinoderm distribution found in this study is very close to that
illustrated by Caddy (1970). Solaster papposus, Henricia spp., Ophiura sarsi, Cucumaria
frondosa, and Strongylocentrotus have all been described as having the ability to survive
strong flow and abrasion (Ursin 1960). All require hard substrate to attach to as movement of
tube feet over soft substrate is difficult. The most frequently observed sea cucumber was
Cucumaria frondosa, the orange-footed sea cucumber. This species was associated with
rocky substrates in all three study sites and is being commercially harvested in New England.
Other species of holothuroidians were collected.
Chordata: Many more species of ascidians, commonly referred to as tunicates or sea
squirts, were observed in the Grand Manan site than in any other site. The stalked ascidian
Boltenia ovifera was locally abundant, clogging the gear during one tow. Several specimens
were observed to have sponges, hydroids, and other epifaunal species encrusting the stalk.
Cliona intestinalis was observed colonizing the insides of dead P. magellanicus shells.
EPIFAUNAL OCCURRENCES ON SCALLOP SHELLS
Caddy (1970) suggests that the distribution of epifaunal species occurring on P. magellanicus
shells is directly proportional to the distribution of the scallop. No epifaunal species recorded
in this study occurred exclusively on the sea scallop. The larger scallops tended to have a