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Predicting interactions between recolonising marine
mammals and fisheries: defining precautionary
management
A. A. AUGE
´
School of Surveying/Department of Zoology, University of Otago, Dunedin, New Zealand
ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Australia
A. B. MOORE
School of Surveying, University of Otago, Dunedin, New Zealand
B. L. CHILVERS
Aquatic and Threats Unit, Department of Conservation, Wellington, New Zealand
Abstract Interactions between fisheries and marine mammals have created costly and unresolved issues throughout
the world. This study examines the spatial and resource overlaps between recolonising New Zealand sea lions,
Phocarctos hookeri (Grey) (using satellite tracking) and local fisheries (using spatio-temporal catch database) on the
Otago coast, New Zealand. Around Otago, spatial and resource overlaps existed year-round and it is predicted that
incidental deaths in fishing gear and resource competition may arise as the sea lion population increases. Preventive
management methods (e.g. marine protected areas) and monitoring studies (e.g. fish stock assessments) are proposed.
The use of precautionary management could ensure sustainable profitable fisheries and successful recolonisation by
sea lions around Otago, and it could be used as a case study for other areas with recovering marine mammal
populations that interact with fisheries.
KEYWORDS: bycatch, competition, fishery management, marine protected area, Otago Peninsula, seals.
Introduction
Fisheries and marine mammals often have overlapping
ranges as they both target similar species, especially in
coastal areas. Three main interactions arise from this
spatial overlap: incidental deaths (bycatch) of marine
mammals in fishing gear, depredation (predation of fish
in fishing nets; both direct interactions) and competition
for resources (indirect interaction). Bycatch of marine
mammals in fisheries and depredation are worldwide
issues, and competition is thought to exist in several
marine areas (Read 2008; Kovacs et al. 2011). All are
detrimental to fisheries owing to damaged gear (inclu-
ding for recreational fishing) and lost time during opera-
tions. Restrictions of fishing activities to limit bycatch of
marine mammals also lead to loss of revenues by fisher-
ies (Bisack 2008). Depredation by marine mammals and
damage to nets can also cause major fishing losses
(Wickens 1996; Varjopuro 2011) and has led to illegal
killing of marine mammals directly during fishing activi-
ties (Loughlin & York 2000). Bycatch in commercial
and recreational fishing activities also poses a threat to
the sustainability of many marine mammal populations
(Read 2008). Consequently, there are numerous incen-
tives for both fishers and conservationists to avoid
bycatch and depredation.
Competition for resources between fisheries and
marine mammals is more difficult to assess than direct
interactions as it requires data on diet, energetic require-
ments and amount of fish taken by marine mammal
Correspondence: A. A. Augé, School of Surveying/Department of Zoology, University of Otago, Dunedin, New Zealand (e-mail: amelie.auge@
gmail.com)
© 2012 Blackwell Publishing Ltd. doi: 10.1111/j.1365-2400.2012.00861.x
Fisheries Management and Ecology, 2012
1
Fisheries Management
and Ecology
populations combined with fishery catches. It has been
the centre of debate in several parts of the world as
resource overlaps (i.e. marine mammals and fisheries tar-
geting the same fish species) often exist (Wickens et al.
1992; Bjørge et al. 2002; Atkinson et al. 2008). Recrea-
tional fisheries can also have significant impacts on fish
stocks and be involved in resource competition with
marine mammals (Coleman et al. 2004). Competition
may be increased by fishing activities damaging or
destroying benthic habitats that support marine mammal
resources (Friedlander & DeMartini 2002).
Deaths of threatened New Zealand sea lions, Phocarc-
tos hookeri (Grey), as bycatch in commercial fisheries
around the Auckland Islands, New Zealand (50.4°S,
166.3°E), have been a long, costly and controversial
issue since the 1980s (Chilvers 2008). The Auckland
Islands host two of the three breeding areas of this spe-
cies, since being extirpated from North and South
Islands of New Zealand by subsistence hunting and seal-
ing (Childerhouse & Gales 1998). The seasonal squid
fishery regularly catches Phocarctos hookeri in trawl
nets and possibly competes with them (Robertson &
Chilvers 2011). Numerous and expensive studies and
management measures have been undertaken, incurring
large economic losses to the fishing industry and large
cost to the New Zealand government (West et al. 1999;
Breen et al. 2003; Chilvers 2008), but have been unsuc-
cessful at mitigating the issue (Robertson & Chilvers
2011).
An initial population of breeding Phocarctos hookeri
has established itself on the Otago Peninsula, South
Island, New Zealand (45.5°S, 170.4°E) since 1994
(McConkey et al. 2002). This is currently the first recol-
onisation attempt by this species onto the South Island
of New Zealand. The population is increasing at a slow
rate as it was initiated by a single female (as of 2012
there were only approximately 12 live breeding adult
females, all descended from that female). However, the
increase is exponential and an estimated population of
250 animals should live on the Otago Peninsula by
2031, rising to 1000 animals by 2044 (Lalas &
Bradshaw 2003). These numbers could be reached faster
if other immigrating females join the population and also
start breeding there. Bycatch in fishing activities (com-
mercial or recreational) affects all marine mammals and
seabirds that occur around the Otago Peninsula in higher
numbers than the current population of sea lions
(Dawson 1991; Darby & Dawson 2000; Bremner et al.
2009). Female Phocarctos hookeri around Otago forage
in areas where fishing occurs (mostly in two critical hab-
itats, shallow rocky reefs and bryozoan thickets) and
prey on commercially and recreationally important fish
species (Augé et al. 2011a,b). Consequently, there may
be risks that bycatch in and competition with fisheries
will arise for Phocarctos hookeri around Otago when
sea lion numbers increase, possibly causing socio-eco-
nomic problems and/or impeding the recolonisation pro-
cess within 20 or 30 years without management
planning. Identifying these risks may help produce pre-
cautionary management measures that will avoid a situa-
tion where detrimental interactions between fisheries and
sea lions arise in the future.
This study presents evidence that such detrimental
interactions will start as the recolonisation by sea lions
progresses and more animals forage in the area. It pro-
poses management options that could progressively be
implemented by fishers and conservationists to avoid
conflict. The study aimed to determine the spatial over-
lap between foraging areas of recolonising female Phoc-
arctos hookeri around Otago and the spatial and
temporal distribution of commercial and recreational
fisheries. It also aimed to establish the resource overlap
between commercial and recreational fisheries and these
sea lions based on their diet and estimates of their
energy requirements and fishery catches. The results pre-
dict where risks of bycatch and competition will likely
arise in this first recolonising area as the sea lion popula-
tion increases. Finally, precautionary management
options are discussed.
Materials and methods
Spatial overlap
The foraging areas of 13 Otago female Phocarctos hook-
eri (including all adults around Otago) were studied dur-
ing the autumns of 2008, 2009 and 2010 using satellite
telemetry (see Augé et al. (2011a), and Augé (2010) for
details on methods and analyses). Foraging areas in
other seasons were not investigated because of financial
and logistical constraints but these animals resided
almost permanently on or close to the Otago Peninsula
from January 2008 to June 2010 (the study period) and
exhibited site fidelity for foraging areas between years
(as presented in Fig. 1 and in more detail in Augé
2010). Consequently, they are likely to use the same for-
aging areas year-round and the spatial overlap between
foraging areas and fishing activities can be determined
accurately.
Spatial commercial fishing data were made available
by the New Zealand Ministry of Fishery via the Ware-
hou Catch Effort database. All commercial fishing
vessels of 6 m in length and over are required by the
New Zealand government to report their activities using
specific forms where at least date, latitude and longitude,
type of gear used, duration of fishing and species caught
© 2012 Blackwell Publishing Ltd.
A. A. AUGE
´ET AL.
2
and associated weights must be provided. Three out of
five fishing methods used in the fishing zone around the
Otago Peninsula were known to result in bycatch of sea
lions and were used in further analyses (i.e. trawling,
seining and long-lining; Shaughnessy et al. 1981; Read
2008; Bremner et al. 2009).
Spatial overlap between fishing activities and foraging
areas of sea lions was calculated for the period January
2008 to June 2010. Spatial intensity of fishing activities
was mapped using the duration of fishing activities. The
Warehou Catch Effort data were reduced to start latitude,
longitude and duration of each fishing event. These data
were mapped in ArcGIS (ESRI, Redlands, CA, USA) by
producing a 5-km-resolution raster (to reduce the preci-
sion of data as required by the Warehou database proto-
col) where each cell had the value of the sum of all
duration of fishing events (of the three methods with
potential for bycatch) that took place therein. The level
of annual consistency in the distribution of fishing
activities was assessed by exploring fishing distribution
during seasons each year. Spatial overlap between forag-
ing areas and fishing activities was visually represented
by producing 65 and 95% Kernel ranges of foraging
areas (volume contours representing the percentages of
the Kernel probability density distribution of the forag-
ing locations) for all sea lions satellite-tracked. These
Kernel ranges were superimposed on the maps of fishing
distribution.
Spatial recreational fishing data were not available,
although recreational fishing activities usually take place
in coastal waters within 3 km from shore (Teirney &
Kilner 2002). Most of the fishing activities reported by
recreational fishers did not involve a risk of bycatch of
marine mammals, except gillnetting (Teirney & Kilner
2002).
Resource overlap
The diet of male Phocarctos hookeri around Otago is
presented in Lalas (1997) and female Phocarctos hookeri
around Otago in Augé et al. (2011b). To estimate the
food requirement by mass for sea lion populations, equa-
tions for energetic requirement of sea lions presented in
Winship et al. (2002) were used, with a proportion of
time on land of 50% based on Augé et al. (2011a). The
annual food requirement of the current population of
Phocarctos hookeri around Otago Peninsula and that of
a hypothetical population of 1000 animals (predicted to
be reached by 2044) was determined using the average
energy content of sea lion diet and the food require-
ments, with sex and age group adjustment.
The average annual amount of fish taken by commer-
cial fisheries was calculated from the Warehou Catch
Effort database from data obtained between January
2008 and June 2010. Only fishing events taking place
within the foraging area of sea lions were used for anal-
yses. This foraging area was defined as the minimum
convex polygon of all foraging locations of sea lions
with a 15-km buffer to accommodate a minimum of fish
stock movement. This area extended over 4953 km
2
.
The amount taken by fisheries was only calculated for
the top 10 fish species found in the diet of male and
female Phocarctos hookeri (Lalas 1997; Augé et al.
2011b) and corresponding to over 90% of the diet by
mass of each sex.
The average annual amount of fish taken by recrea-
tional fisheries within the foraging area of sea lions was
obtained from Teirney and Kilner (2002). This report
contains estimated number of fish caught by recreational
fishers within defined areas of the New Zealand coastline
based on an annual survey of a sample of fishers. Esti-
mated annual numbers of fish (only fish found in the diet
Figure 1. Concurrent overlaps between commercial fishing activities
(trawling, seining and long-lining) and foraging areas of female New
Zealand sea lions born at the Otago Peninsula, during 2008 (n=4),
2009 (n=7), and 2010 (n=6) autumns. Each fishing cell is
595 km.
© 2012 Blackwell Publishing Ltd.
FISHERIES, MARINE MAMMALS: PRECAUTIONARY MANAGEMENT 3
of Phocarctos hookeri were used in the analyses) taken
within the coastal area included in the foraging area of
sea lions were used for calculation. These numbers pro-
vided a value for the annual amount of fish in mass
taken by recreational fishers based on the most common
length and weight of a fish of each species found in
FishBase (2011). The combined amount of fish taken
annually by commercial and recreational fisheries within
the foraging area of sea lions was used to compare
against the amount taken by sea lions and assess the
potential for competition.
Results
Spatial overlap: bycatch risk
Commercial fishing activities that have the potential to
catch Phocarctos hookeri incidentally comprised on
average 76% of trawling operations. Cells of 5-km
resolution included in the foraging area of female Phoc-
arctos hookeri annually experienced up to 244 h of fish-
ing activities susceptible to bycatch. Each year, fishing
activities were similarly distributed during each season
as shown in the example for distribution of fishing acti-
vities during three autumns (Fig. 1).
Fishing activities were unequally distributed around
the Otago Peninsula. The area north of the Otago Penin-
sula, where fishing activities were concentrated, was the
area with the highest level of spatial overlap between
fisheries and sea lions (Fig. 2). Smaller areas of overlap
also existed in the south and east of the Otago Peninsula
(Fig. 2).
Resource overlap: competition risk
Main prey species of Phocarctos hookeri around the
Otago Peninsula and their energy contents are presented
in Table 1. The average energy content of Phocarctos
hookeri diet, weighted by the percentage in the diet of
the main species, is 6.2 kJ g
1
for males and 7.3 kJ g
1
for females. Values of energetic requirements for the
Otago sea lion population are summarised in Table 2.
The current population of Phocarctos hookeri based
year-round around the Otago Peninsula is approximately
75 males and 22 females and their annual estimated food
requirement is 246 t (Table 3). For a population of 1000
individuals, this value rises to 2588 t yr
1
(Table 3).
Fisheries exploiting the zone corresponding to the Otag-
o sea lion foraging area caught annually 377 t of fish
(76.7% commercially) that are known to be in the diet of
Phocarctos hookeri (Table 4). This component of the
commercial catch represented only 28.2% of the total
1022 t of fish caught commercially each year. Some fish
species are almost exclusively caught by recreational fish-
ers, in amounts close to that of commercial fisheries, for
example, blue cod, Parapercis colias Forster (Table 4).
Discussion
The potential for bycatch, depredation and competition
risks between fisheries and Phocarctos hookeri around
the Otago Peninsula that may become critical within a
10–20 years timeframe was identified. There has been
no reported Phocarctos hookeri death as bycatch in fish-
ing activities around the New Zealand mainland because
the number of sea lions is currently very low. However,
considerable potential overlap exists between fishing
activities and areas where female Phocarctos hookeri
forage. As sea lion numbers increase around the Otago
Peninsula, bycatch may become an issue similar to that
of recolonising New Zealand fur seals (Gibson 1995).
Depredation has not been reported around Otago,
although some recreational fishers have complained in
the local media about sea lions interacting with their
fishing gear and taking their fish. As more sea lions
inhabit the Otago coast, depredation issues with the
commercial fishery may also arise. Regarding competi-
tion, the fishing quota system in New Zealand is set to
harvest fish stocks to 40% of their estimated unfished
biomass (Ministry of Fisheries 2011). Assuming that the
catches reported in this study corresponded to only half
of the 60% allowed to be caught (owing to the small
Figure 2. Spatial overlap between foraging areas of female New
Zealand sea lions (n=13) and commercial fishing activities with
potential bycatch of sea lions (trawling, seining and long-lining) around
the Otago Peninsula, represented by the number of hours of fishing in
595 km cells (period January 2008 to June 2010).
© 2012 Blackwell Publishing Ltd.
A. A. AUGE
´ET AL.
4
spatial scale at which sea lions forage, it is not possible
to determine this value as quotas are given for much
larger areas), combined catch by fisheries and a popula-
tion of 1000 sea lions may reduce fish stocks to only
approximately 10% of their unfished biomass by 2040
at the latest. At this level, a fish stock is designated as
‘collapsed’(Ministry of Fisheries 2011). Consequently,
competition will arise in that scenario.
The situation of Phocarctos hookeri around Otago can
be compared with that of grey seal, Halichoerus grypus
(Linnaeus), in the Baltic Sea in the late 1960s (Varjopuro
2011). These seals were then considered at risk of
extinction and all conservation funds and actions were
concentrated on their recovery without anticipation of
the consequences for fisheries. The population of Phoc-
arctos hookeri around Otago is currently small and vul-
nerable. However, if the recolonisation process is
successful (i.e. a breeding colony establishes), the popu-
lation will exponentially increase within the next few
decades as with the grey seal. In the Baltic Sea, Halic-
hoerus grypus are now engendering large economic
losses to coastal fisheries through depredation and man-
agement plans are only currently being developed by the
Finnish government (Varjopuro 2011). However, these
Table 1. Main prey species of New Zealand sea lions around the Otago Peninsula and corresponding energy contents
% in male diet % in female diet Energy content (kJ g
1
) References
Thyrsites atun (Cuvier) 25 26 6.9 Vlieg (1984)
Trachurus sp. 14 31 7.6 Pickston et al. (1982)
Macroctopus maorum (Hutton) 13 - 3.8 Meynier et al. (2008)
Table 2. Values used to calculate the food requirement by mass of Otago New Zealand sea lions. GER: Gross Energy Requirement (from Winship
et al. 2002)
Average
mass (kg)
†
GER
(MJ day
1
)
Estimated amount of food
required (kg day
1
)
‡
Current number in
recolonising population
§
Estimated percentage in an
established population
¶
Adult male 279 65.8 10.6 30 30.0
Non-lactating
adult female
125 36.1 4.9 8 12.3
Lactating adult
female
125 61.3 8.4 6 17.7
Juvenile male (1–
3 year)*
110 32.8 5.3 45 20.0
Juvenile female (1
–3 year)*
87 27.5 3.8 8 20.0
*
Pups (<1 year) are considered as fully dependant on lactating females for food.
†
From Geschke and Chilvers (2009) for males and Augé et al. (2011c) for all others except for juvenile males (estimated).
‡
Based on average prey energy content of 6.2 kJ g
1
for males and 7.3 kJ g
1
for females.
§
From Augé (2010).
¶
On the basis of reproduction rate of 0.67 pup yr
1
(Childerhouse et al. 2010) and 12.2% pup death rate at 3 months (Chilvers et al. 2007) for
adult females, and estimated from Holmes et al. (2007) for juveniles.
Table 3. Estimated amount of food required by New Zealand sea lions at Otago for the current population size and for a population size of 1000
animals, given per day and per year
Current population Population size of 1000
No. animals Food per day (in kg) Food per year (in t) No. animals Food per day (in kg) Food per year (in t)
Adult male 30 318 116 300 3183 1162
Non-lactating adult female 8 40 14 123 608 222
Lactating adult female 6 50 18 177 1486 543
Juvenile male 45 238 87 200 1058 386
Juvenile female 8 30 11 200 753 275
Total 246 2588
© 2012 Blackwell Publishing Ltd.
FISHERIES, MARINE MAMMALS: PRECAUTIONARY MANAGEMENT 5
plans could have been discussed and researched before
the problem started to avoid current associated costs to
fisheries. Around Otago, precautionary fisheries manage-
ment strategies should be designed and introduced to
avoid the development of situations such as that of the
Baltic Sea Halichoerus grypus.
The further recolonisation of the Otago coast by
Phocarctos hookeri and the related exponential increase
in their numbers will likely onset considerable issues
for fisheries in potentially as little as 10–20 years.
However, various precautionary management strategies
along with the production of a detailed management
plan, developed in partnership between fishers and con-
servationists, could be progressively implemented to
anticipate any onset of bycatch or competition issues.
This should include marine protected areas, local fish-
ing quotas and management of sea lion numbers (cur-
rently illegal under the New Zealand Marine Mammal
Protection Act 1978). The first option can be put for
consultation and progressively implemented as soon as
deemed necessary by all parties involved, whilst the
other two options will first require additional data on
marine production, fish stocks and sea lion population
dynamics around the Otago Peninsula.
A marine protected area (MPA) or a network of
MPAs with fishing restrictions or no-take zones would
ensure bycatch, depredation and competition issues are
avoided. MPAs would also protect critical benthic habi-
tats from damage or destruction. Shallow rocky reefs
and bryozoans thickets (the two main foraging habitats
of sea lions) have significant environmental values and
their protection from fishing damage could benefit the
overall ecosystem of the area. Areas of coral formations
such as bryozoans are often associated with higher levels
of biodiversity and biomass than soft sediments and are
important for juvenile fish (Mortensen et al. 1995).
Rocky reefs are an important habitat for juvenile fish in
New Zealand (Andrew & Francis 2005) and are vulnera-
ble to damage by fishing activities (Steneck et al. 2002).
In New Zealand, MPAs are jointly implemented and
administered by the Ministry of Fisheries and Depart-
ment of Conservation. Local fishery parties (commercial
and recreational) are also involved in the decision pro-
cess, along with conservation organisations and other
stakeholders. This makes MPAs the best available man-
agement method to ensure that fishery and conservation
sides are represented and collaborate to determine the
best options.
On the basis of the areas of overlap between fishing
activities and foraging areas of sea lions and on the biolog-
ical importance of bryozoan thickets and coastal shallow
rocky reefs, four different potential areas are proposed for
MPAs (Fig. 3). The area of bryozoan thickets could be
entirely protected from fishing under a no-take zone (i.e.
classified as a marine reserve). Proposed Area 1 had the
highest level of potential direct interactions between sea
lions and fishing activities and should receive the status of
MPA where only fishing gears with no bycatch risk can be
used. As this area supported the highest level of fishing
activities, a no-take zone would severely impede fishers
and would consequently be inadequate. Proposed Area 2
corresponds to the main coastal area used by sea lions
Table 4. Estimated mass of fish taken annually by commercial and
recreational fisheries within the foraging area of the New Zealand sea
lion population around the Otago Peninsula. NS, no significant amount
(<1 t) (based on data obtained from January 2008 to June 2010)
Fish species
Amount taken by
commercial fisheries (t)
Amount taken by
recreational fisheries (t)
Raja nasuta 78.0 NS
Genypterus
blacodes
73.8 NS
Pseudophycis
bachus
60.1 11.7
Nototodarus
sloanii
37.3 NS
Thyrsites atun 33.0 22.2
Parapercis
colias
2.8 43.3
Flatfish (soles
and flounders)
2.2 7.4
Colistium
guntheri
1.6 NS
Odax pullus NS 3.1
Trachurus sp. NS NS
Macroctopus
maorum
NS NS
Squalus
acanthias
NS NS
Wrasse Labridae
2 spp.
NS NS
Total 288.8 87.7
Figure 3. Proposed areas for precautionary management of interac-
tions between local fisheries and New Zealand sea lions around the
Otago Peninsula, New Zealand.
© 2012 Blackwell Publishing Ltd.
A. A. AUGE
´ET AL.
6
where commercial fishing was low but where recreational
fishing occurs. Hence, this area could be designated as an
MPA with a series of no-take zones alternating with zones
where recreational fishing could still be allowed (with a
ban on gillnetting). This would ensure that the two critical
sea lion foraging habitats are fully protected and decrease
risks of competition as marine reserves that protect habi-
tats important for juvenile fish have been shown to
improve fish stocks in neighbouring unprotected areas
(Roberts et al. 2001). Proposed Area 3 could be the buffer
zone. Some bycatch risk exists but if Area 2 and the bryo-
zoan thickets become MPAs, the inclusion of this area as
a marine reserve would allow easier management of one
entity that protects almost all foraging areas of sea lions.
These different areas could be progressively implemented
over the next decade as the sea lion population increases,
leaving time for local fishers to adapt to the changes and,
possibly, make some recommendations for improvements.
Further management options (i.e. local fishing quotas,
sea lion control) will rely on data available to model fish
stocks and marine production around the Otago Penin-
sula, and changes in law. A priority for the management
of interactions between fisheries and sea lions around the
Otago Peninsula is to increase the amounts of data and
knowledge of the local marine ecosystem, especially fish
stocks.
In conclusion, bycatch, depredation and competition
are expected as the population of Phocarctos hookeri
increases on the Otago Peninsula and no management
measures are implemented. Overall, the main area used
by sea lions is relatively small and management mea-
sures would be easy to achieve with minimum economic
losses or disadvantages to fisheries. If this is only imple-
mented after conflicts occur, costs to the economy will
be exponentially larger as demonstrated in the Auckland
Islands and other parts of the world. Consequently, the
introduction of precautionary measures, an explicit man-
agement plan and a research programme into marine
production, fisheries and sea lion population dynamics
around Otago would likely benefit both fisheries and sea
lion conservation. This could test the effectiveness of
precautionary management for future recolonisation
events by sea lions at other sites in New Zealand, but
also for the recovery or recolonisation of marine mam-
mals around the world.
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