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Ecological and evolutionary consequences of size-selective harvesting: How much do we know?

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Ecological and evolutionary consequences of size-selective harvesting: How much do we know?

Abstract

Size-selective harvesting, where the large individuals of a particular species are preferentially taken, is common in both marine and terrestrial habitats. Preferential removal of larger individuals of a species has been shown to have a negative effect on its demography, life history and ecology, and empirical studies are increasingly documenting such impacts. But determining whether the observed changes represent evolutionary response or phenotypic plasticity remains a challenge. In addition, the problem is not recognized in most management plans for fish and marine invertebrates that still mandate a minimum size restriction. We use examples from both aquatic and terrestrial habitats to illustrate some of the biological consequences of size-selective harvesting and discuss possible future directions of research as well as changes in management policy needed to mitigate its negative biological impacts.
Molecular Ecology (2008) 17, 209–220 doi: 10.1111/j.1365-294X.2007.03522.x
© 2007 The Authors
Journal compilation © 2007 Blackwell Publishing Ltd
Blackwell Publishing Ltd
Ecological and evolutionary consequences of size-selective
harvesting: how much do we know?
PHILLIP B. FENBERG and KAUSTUV ROY
Section of Ecology, Behaviour and Evolution, University of California, San Diego, La Jolla, CA 92093-0116, USA
Abstract
Size-selective harvesting, where the large individuals of a particular species are prefer-
entially taken, is common in both marine and terrestrial habitats. Preferential removal of
larger individuals of a species has been shown to have a negative effect on its demography,
life history and ecology, and empirical studies are increasingly documenting such impacts.
But determining whether the observed changes represent evolutionary response or pheno-
typic plasticity remains a challenge. In addition, the problem is not recognized in most
management plans for fish and marine invertebrates that still mandate a minimum size
restriction. We use examples from both aquatic and terrestrial habitats to illustrate some of
the biological consequences of size-selective harvesting and discuss possible future direc-
tions of research as well as changes in management policy needed to mitigate its negative
biological impacts.
Keywords: fishery, invertebrates, macroevolution, microevolution, size-selective harvesting, terrestrial
vertebrates
Received 8 February 2007; revision received 11 July 2007; accepted 1 August 2007
Introduction
Body size is generally considered to be one of the most
important traits of an organism because it correlates with
many aspects of its biology, from life history to ecology
(Peters 1983; Calder 1984). Size-selective harvesting, where
large individuals of a particular species are preferentially
taken, is a common practice in both terrestrial and marine
habitats. Such harvesting practices are not only prevalent
among people taking animals for food and other needs, but
are also mandated by the management plans for many fish,
invertebrate and game species. In fact, evidence for size-
selective harvesting goes back to some of the earliest
archaeological records of human settlement, dating back
to at least the Middle Stone Age (Jerardino et al. 1992;
Siegfried 1994; Mannino & Thomas 2002; Klein et al. 2004).
It is not surprising that large individuals make up the
bulk of the specimens in those archaeological deposits
since they are the easiest to find and give the highest yield
of protein per unit effort (Raab 1992). However, as sub-
sistence and artisanal harvesting have given way to
commercial exploitation and industrial fishing, an increasing
number of species worldwide have been subjected to
size-selective harvesting. There is growing evidence that
decades of size-selective harvesting has led to the reduction
in body sizes of many species and that such artificial
selection against large body size affects not only the
targeted species but also the surrounding community
(see below). However, the effects of size-selective harvesting
are multifaceted and often species and system specific.
Thus, even though size-selective harvesting is increasingly
being recognized as a cause for concern (Birkeland & Dayton
2005), so far the ecological and evolutionary consequences
of this practice have been explored only for a limited
number of species.
In this paper, we first provide an overview of the scope
and nature of size-selective harvesting. In particular, we
estimate the number and types of species that are known to
have been affected by size-selective harvesting practices
and look at how that information has changed over the
last few decades. We then briefly review the effects of size-
selective harvesting on life history, demography and
ecology of the exploited species, and discuss the evolution-
ary consequences of such impacts. A potential complication
here comes from the fact that in the case of some species,
both terrestrial and aquatic, harvesting preferentially
Correspondence: Kaustuv Roy, Fax: (858) 534-7108; E-mail:
kroy@ucsd.edu
210 P. B. FENBERG and K. ROY
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Journal compilation © 2007 Blackwell Publishing Ltd
targets large individuals of a specific sex. For example,
trophy hunters usually take the largest (and oldest) males
in a population and/or those with the most impressive (i.e.
largest) horns, antlers or tusks (Milner-Gulland & Mace
1991; Ginsberg & Milner-Gulland 1994; Coltman et al. 2003;
Milner et al. 2007). Because such harvesting preferentially
targets large individuals, albeit of a specific sex, we include
it under our general definition of size-selective harvesting.
However, it is important to keep in mind that from an
evolutionary standpoint, this practice is different from the
more common one of harvesting large individuals without
regard to their sex, since the traits under direct selection are
different (size in one case, size of a particular sex in the
other). Thus, where appropriate, we have treated these two
types of size-selective harvesting separately. We end the
paper with a brief discussion about fruitful directions for
future research and also changes in management policy
necessary to mitigate the negative biological impacts of
size-selective harvesting.
Literature search
Although size-selective harvesting takes place both on
land and in the ocean, at present it is unclear what
proportion of the biota are affected by this practice. In
order to get a better estimate of the scope and nature of
the problem, we undertook an extensive search of the
literature extending back to 1975 to identify species that are
known to have been affected by size-selective harvesting.
Our search criteria excluded studies that are solely based
on theoretical models or simulations and focused only
on animals. There are many examples of size-selective
harvesting of tree species (e.g. Ledig 1992; Hall et al. 2003),
but for practical reasons, we limited our analyses to animal
species.
We used two different search strategies to estimate the
number of species being affected by size-selective harvest-
ing. Our first approach is conservative in that we only
included species if the author(s) explicitly stated that
exploited populations (species) are size-selectively harv-
ested or if they used some variation of the following
phrases: the large individuals are preferentially harvested;
the large size classes experience higher harvest mortality;
changes in life history are a response in part due to the
size-selective nature of the harvest regime; the larger sex
of a sequentially hermaphroditic or sexually dimorphic
species is preferentially harvested.
Our second approach is more comprehensive in that we
also included species for which there is only indirect evid-
ence for size-selective harvesting, such as a reduction
in mean or maximum body size through time or between
exploited and protected areas. Under heavy harvesting
pressure, even when it is not necessarily size selective, a
truncation of the largest (oldest) size classes of a population
is expected (Trippel 1995; Law 2000; Heino & Godo 2002),
because of which, it can be difficult to distinguish the
effects of overharvesting per se from size-selective harvest-
ing just by comparing size-frequency distributions. In
practice, we found this issue to be relevant primarily for
marine fish, where many species and populations are
under intense harvesting pressure. Despite this compli-
cation, we include the results of this more comprehensive
search not only because it includes species potentially
missed by the previous search but also because, strictly
speaking, almost all fisheries are inherently size selective
by nature (Policansky 1993; Law 2000; Heino & Godo 2002;
see below).
It is important to note that the aim of our literature search
was to identify a representative set of papers that would
permit an unbiased evaluation of the taxonomic scope and
the nature and consequences of size-selective harvesting,
rather than an exhaustive list of all publications on size-
selective harvesting. We restricted our search to the
peer-reviewed journal articles and ignored sources such as
technical reports.
In order to compare the patterns across different groups
we classified each species into one of three general cat-
egories: terrestrial vertebrates, fish (freshwater and marine)
and marine invertebrates. For our analyses of temporal
trends we only used the earliest publication date that
reported size-selective harvest for each species. For
example, five separate papers in our data set report size-
selective harvesting of the limpet Lottia gigantea but only
the earliest of these, Pombo & Escofet (1996), is used for this
analysis.
How pervasive is size-selective harvesting?
The conservative literature search identified a total of 108
species of fish, invertebrates and terrestrial vertebrates
that are known to have been subjected to size-selective
harvesting pressure. Of these, 87 are aquatic (marine and
freshwater) and 21 are terrestrial. The aquatic taxa include
48 species of fish (freshwater and marine) and 39 species
of invertebrates (only marine). Ungulates make up the
majority of terrestrial examples (15 species), the rest are
carnivores/omnivores (4 species), elephants and kangaroos.
The only difference between the conservative and the
more comprehensive estimate is that the latter increases
the number of size-selectively harvested fish species to 76,
raising the total number of species to 136.
Commercial and artisanal harvesting involves many
more species of fish and shellfish than terrestrial verte-
brates, which is reflected in the very strong bias towards
aquatic species seen in our data. Thus, in terms of species
numbers, size-selective harvesting is primarily a concern
for aquatic species, especially those living in the ocean,
and to a lesser extent for some large terrestrial vertebrates.
EFFECTS OF SIZE-SELECTIVE HARVESTING 211
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In the latter case, harvesting is usually both sex and size
selective. While the bias towards aquatic species is likely
to be real, our search also revealed that the scientific
literature on size-selective harvesting is still relatively
limited, and does not capture the true scope of the problem.
The selective loss of larger individuals is an inevitable
consequence of most commercial and recreational fisheries
(Sluka & Sullivan 1998; Beard & Kampa 1999; Law 2000;
Law 2001; Longhurst 2006), which suggests that the number
of fish species estimated to be affected by size-selective
harvesting even by our comprehensive search is too low.
In addition, illegal poaching of terrestrial vertebrates also
tends to be size selective (Milner-Gulland & Mace 1991;
Milner-Gulland et al. 2003), but again our knowledge of
the biological consequences of such harvesting is pres-
ently limited to only a handful of species. However, the
number of exploited species for which we have some
information in the peer-reviewed literature has been
increasing over the last couple of decades (Fig. 1). The
increase is evident for all the groups in our database, but is
particularly strong for fish and invertebrates (Fig. 1).
Furthermore, the trend is qualitatively the same whether
we use the conservative or the comprehensive search
(Fig. 1a).
The mechanisms used by commercial, recreational
and artisanal fisheries and hunters to preferentially
remove large individuals are almost as diverse as the
number of species affected by such harvesting practices.
For example, commercial fisheries tend to select larger fish
through the use of different kinds of fishing gear such
as trawls and gillnets (based on mesh size), longlines and
trap nets (Bohnsack et al. 1989; Policansky 1993; Dahm
2000; Law 2000), while some recreational fisheries, such
as spear fishing for groupers in the Florida Keys, involves
searching for individuals over certain size thresholds
(Sluka & Sullivan 1998). Actively searching for and select-
ing large individuals is also common when people harvest
marine invertebrates such as abalone and limpets (Branch
& Moreno 1994; Lindberg et al. 1998; Murray et al. 1999;
Moreno 2001), or hunt large terrestrial vertebrates
(Ginsberg & Milner-Gulland 1994). Interestingly enough,
one of the largest sport hunting activities in the USA
tends to preferentially remove the smaller rather than the
larger individuals in a population. Duck hunters generally
shoot individuals of lower overall condition (i.e. lower
body mass) because they are more abundant at feeding
decoys (Weatherhead & Ankney 1984). This is exactly the
opposite of the trend seen for most other species where
the largest and presumably most fit individuals are
preferentially hunted. In fact, there are many documented
examples of increased hunting mortality of lower con-
dition ducks (Greenwood et al. 1986; Hepp et al. 1986;
Reinecke & Shaiffer 1988; Dufour et al. 1993; Heitmeyer
et al. 1993).
Consequences of size-selective harvesting
Size-selective harvesting can affect many aspects of the
biology of an organism, from life history, demography,
genetics and behaviour to the local abundance and biomass
of populations. One of the biggest challenges to under-
standing the biological effects of size-selective harvesting
is that many of the details tend to be taxon-specific. In some
cases, it can also be difficult to distinguish between the effects
of heavy but not necessarily selective exploitation and
size-selective harvesting. As mentioned above, truncation
of the large size classes and consequent changes in life
history and demography are expected under both of these
situations (Conover 2000; Heino & Godo 2002). For species
Fig. 1 (a) A plot of the cumulative number of fish species
(marine and freshwater) reported to be affected by size-selective
harvesting since 1975. The data points marked by closed circles
represent the temporal accumulation of size-selectively harvested
species using our comprehensive approach and those marked by
open circles represent the results of our conservative approach
(see text for details). Each species was added to the plot based on
the earliest record (publication date) in our database of it being
subjected to size-selective harvesting. (b) A plot of the cumulative
number of marine invertebrate and terrestrial vertebrate species
reported to be affected by size-selective harvesting since 1975. See
text for details of the search protocols.
212 P. B. FENBERG and K. ROY
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Journal compilation © 2007 Blackwell Publishing Ltd
where we have information about harvesting practices,
it is relatively straightforward to determine whether a
population has been subjected to size-selective exploitation.
For example, the largest individuals of many intertidal
invertebrate species are selectively harvested for food since
they provide the most meat for the effort (Siegfried 1994).
Similarly, trophy hunters almost always target large
males (Ginsberg & Milner-Gulland 1994; Coltman et al.
2003; Milner et al. 2007). But in the case of commercially
harvested marine fish, it can sometimes be difficult to
determine how much of the observed changes in life history
and demography are due to intense exploitation vs. increased
fishing mortality of the largest size classes (i.e. size-
selective harvesting). Most fisheries only catch individuals
above a minimum size that is mandated by fisheries managers
or determined by the gear (e.g. mesh size). Furthermore,
historical data show that almost all fisheries start out by
preferentially harvesting the large individuals (Jennings
& Kaiser 1998; Jackson et al. 2001). Thus, in a strict sense,
most commercial marine fisheries harvest individuals in
a size-selective manner (also see Policansky 1993; Heino
& Godo 2002). However, many marine species are currently
under intense harvesting pressure and the minimum size
mandated by management plans may be quite small
relative to the maximum sizes that have been historically
attained by individuals in exploited populations (see
Jackson et al. 2001). In these species, most size classes rather
than just the largest ones experience elevated mortality
due to fishing, which can lead to changes in life history
and demography. Clearly, defining what constitutes
size-selective harvesting is somewhat arbitrary and it is
important to recognize that most harvesting strategies can
lead to increased adult mortality rates and a reduction in the
number of large individuals present in a population. In the
discussion below, where appropriate, we have attempted
to separate the effects of size-selective harvesting per se from
those due to intense exploitation (or overharvesting) but in
some cases, it may be difficult to disentangle the two.
Changes in body size and mortality rate
The primary effect of size-selective harvesting and
exploitation in general is an overall reduction in body size
and an increased mortality rate of the harvested species.
Body size declines attributed either partly or primarily to
size-selective harvesting have now been documented
in many species of marine and freshwater fish (e.g.
Ricker 1981; Beard & Kampa 1999; Zwanenburg 2000;
Harvey et al. 2006), marine invertebrates (e.g. Branch 1975;
Siegfried 1994; Moreno 2001; Branch & Odendaal 2003;
Roy et al. 2003) and some terrestrial vertebrates (e.g.
Coltman et al. 2003). When size-selective harvesting targets
a particular sex, reductions in body size are evident only
for that sex. For example, hunting of bighorn trophy rams
leads to a significant reduction in body size of males in the
population (Coltman et al. 2003). In most cases, relatively
little information exists about how long size-selective
harvesting has been taking place or the rate of decline over
time. Quantifying the rate of decline requires historical
information about body sizes (Jackson et al. 2001) and
although such information is potentially available for
many species of marine invertebrates, so far only a few
studies provide such analyses (e.g. Roy et al. 2003). In
contrast, many studies of exploited fish species provide
information on how sizes have declined over time (e.g.
Handford et al. 1977; Ricker 1981; Zwanenburg 2000;
Jackson et al. 2001; Harvey et al. 2006; Hsieh et al. 2006).
A decline in body size because of exploitation is also
associated with changes in the mortality schedule of
affected populations. There are two sources of mortality
that natural populations have evolved with, intrinsic and
extrinsic (Stearns 1992). Intrinsic sources of mortality are
those that contribute to patterns of senescence and ageing,
whereas extrinsic sources of mortality are associated
with factors such as predation. Human harvesting acts
to increase the extrinsic sources of mortality in affected
populations to such an extent that for many species, it is the
most common cause of adult mortality (Heino & Godo
2002; Festa-Bianchet 2003). Size-selective harvesting by
humans is therefore a source of extrinsic mortality where
the larger size classes experience higher harvest mortality
than the smaller size classes. Although any increase in total
mortality rate is expected to have an influence on life-history
traits (Stearns 1992; Conover 2000), the response is likely to be
more extreme when harvest mortality is size (age) specific
(Stokes et al. 1993; Ginsberg & Milner-Gulland 1994; Conover
2000; Law 2000; Moreno 2001; Heino & Godo 2002; Milner
et al. 2007). Thus, the net result of harvest-induced elevated
mortality is an overall decline in the number of individuals
surviving to older ages and larger sizes, which can lead to
a multitude of cascading effects (see below).
Growth and survival of offspring
The quality of offspring is perhaps the most nonintuitive
trait to be affected by size-selective harvesting, yet there is
increasing evidence for such an effect in both marine and
terrestrial species. The size and quality of larvae of some
exploited marine fish has been shown to be positively
correlated with maternal length and age (Berkeley et al.
2004). For example, older mothers of Sebastes melanops
(black rockfish) provide larger oil globules for their larvae
than younger and smaller females, which can enhance the
growth rate and survival of the larvae (Berkeley et al. 2004).
Similarly, older (larger) females can produce higher quality
eggs, leading to enhanced survival of their larvae (Trippel
1995; Kjesbu et al. 1996; Vallin & Nissling 2000). Given these
and other maternal effects, removal of the oldest and largest
EFFECTS OF SIZE-SELECTIVE HARVESTING 213
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Journal compilation © 2007 Blackwell Publishing Ltd
females has the potential to affect the size, growth and
survival of larvae of a number of fish species (Vallin &
Nissling 2000; Berkeley et al. 2004; Birkeland & Dayton
2005; Longhurst 2006). However, whether such maternal
effects on larval quality are present in most fish species or
whether they are clade specific remains unknown. Similarly,
while maternal size has been shown to influence larval
survival in some marine invertebrates such as bryozoans
(Marshall & Keough 2004), it is unclear how common these
characteristics are in invertebrate species that are exploited
for human consumption.
Among terrestrial animals, some sex-selectively (and
hence size-selectively) hunted ungulate populations show
a reduction in offspring weight. For moose (Alces alces) and
reindeer (Rangifer tarandus), reduced offspring weight may
occur when females are forced to mate with young males
after hunting reduces the number of older males (Saether
et al. 2003; Holand et al. 2006; Milner et al. 2007). This reduc-
tion is not a direct result of lower male physiological health
but an indirect result of delayed parturition dates for off-
spring sired by younger males (Milner et al. 2007). Female
behavioural avoidance of less mature young males early in
the season drives lower calf weight at birth later in the
breeding season (Holand et al. 2006), which is a direct result
of there being fewer large (older) males in the population
because of sex-selective harvesting pressure. In the case of
both grizzly bears (Ursus arctos) and African lions (Panthera
leo), large and socially dominant males are also selectively
hunted as trophies. This can result in an unfortunate
side-effect of reduced survival among juveniles triggered
by an increase in infanticidal behaviour by less dominant
males (Swenson et al. 1997; Whitman et al. 2004; Loveridge
et al. 2007). Once the dominant male has been removed
from the population, the younger and peripheral males seek
to increase their fitness by killing the offspring of the hunted
dominant male and thus, reducing the interbirth period
required to sire the next litter of cubs (Milner et al. 2007).
Reproductive investment
For long-lived species with low natural adult mortality
rates, size-selective harvesting may cause shifts in life-
history traits that are linked to adult survival, such as
reproductive investment. The expected contribution to
population growth of an individual (reproductive value)
changes with age such that juveniles have low reproduc-
tive value (RV), mature adults have high RV, and in long-
lived organisms, RV declines slowly with age (Kokko et al.
2001). It is therefore not surprising that size-selective
harvesting of mature adults with the highest RV can lead
to a decline in population growth rate. The most likely
consequence of high adult mortality through harvest
pressure is an increase in reproductive investment of
young adults (Festa-Bianchet 2003). For hunted ungulate
populations, subadult males are more likely to reproduce
at a younger than normal age because of the reduced
number of competitive older males during the rutting
season (Milner et al. 2007). The effects of size-selective
harvesting on the reproductive investment of invertebrate
species are very poorly studied. But indirect evidence
suggests that at least for some sex- and size-selective crab
fisheries, where large males are preferentially taken, the
surviving younger and smaller males attempt to reproduce
more frequently than they normally would in the presence
of larger and older males (Carver et al. 2005).
Growth and age (size) at maturity
Nearly all of the studies looking at the effects of size-
selective harvesting on growth and age (size) at maturity
have focused on fish (e.g. Handford et al. 1977; Spangler
et al. 1977; Ricker 1981; Stokes et al. 1993; Law 2000; Heino
& Godo 2002; Engelhard & Heino 2004; Baskett et al. 2005).
Although a reduction in the age at maturity of harvested
fish stocks is well documented, particularly for species
with relatively late maturation times (Haug & Tjemsland
1986; Bowering & Brodie 1991; Rijnsdorp 1993; Trippel
1995; Rochet 1998; Law 2000; Grift et al. 2003), the underlying
processes are not always clear (for an excellent review on
this subject see Heino & Godo 2002). For example, fishing
may indirectly select for either an increase or decrease
in individual growth rates depending on any number of
factors, including the size-selective nature of the harvest
regime (Heino & Godo 2002). The size selectivity of the fishing
gear and/or minimum size restrictions may preferentially
remove faster growing individuals that ‘recruit’ to the
fishery at a younger age, leaving individuals with a genetic
tendency to grow more slowly. While some laboratory
experiments have provided support for this hypothesis
(Conover & Munch 2002), the extent to which selection for
slow growth affects maturation times remains poorly known
for a number of reasons. First, fishing, whether size selective
or not, will reduce the stock abundance and decrease the
total number of intraspecific competitors for food resources
(Policansky 1993). This improvement in food access
may result in accelerated juvenile growth and an overall
younger age at maturation (Policansky 1993; Trippel 1995;
Law 2000; Heino & Godo 2002; Engelhard & Heino 2004),
potentially dampening any effects of selection for slower
growth. Furthermore, harvesting-induced elevated mortality
by itself (whether selective or not) is expected to lead to a
very small number of individuals surviving to old ages
and large sizes, leaving relatively young individuals to
dominate the population. Individuals with a tendency to
mature at an early age will contribute more of their genes
to the next generation than individuals with a tendency to
mature at older ages simply because of their probability
of successfully reproducing before being harvested (Law
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2000; Heino & Godo 2002; Engelhard & Heino 2004). This
concern that harvesting might cause genetic changes in
growth or maturation times for exploited fish stocks was
first put forth by Miller (1957) and later by Spangler et al.
(1977), Handford et al. (1977) and Borisov (1978), but was
not intensively examined until the early 1990s (Rijnsdorp
1993; Stokes et al. 1993). More recent studies have implicated
genetic change as being partly responsible for observed
changes in maturation times of some heavily exploited fish
stocks (Grift et al. 2003; Barot et al. 2004; Olsen et al. 2004).
But for most species, it remains unclear to what extent
the observed changes in maturation time are due to
phenotypic plasticity or evolutionary change (Grift et al.
2003). In addition, physical factors such as increased
surface water temperatures caused by global warming can
also contribute to accelerated juvenile growth rates and
associated changes in life history (Thresher et al. 2007).
Finally, because growth typically slows after maturation, a
younger maturation time should result in a smaller size at
age in the future (Heino & Godo 2002). Thus, regardless of
the specific cause, a reduction in the age at sexual maturity
of exploited fish stocks is likely to be followed by an overall
reduction in yield (Law 2000; Conover & Munch 2002;
Heino & Godo 2002; Ernande et al. 2004).
Whether these effects seen in fish also apply to other
marine organisms remain unclear at present. As men-
tioned above, comparable studies of marine invertebrates
are scarce, but studies of exploited marine limpets in South
Africa and Costa Rica show surprisingly little impact of
harvesting on growth rates and age at maturity (Ortega
1987; Branch & Odendaal 2003).
Fecundity and biomass
In fish and invertebrates, fecundity not only increases with
size (Kido & Murray 2003; Birkeland & Dayton 2005) but in
many species, relative fecundity (i.e. fecundity per gram of
body weight) can be higher in older and larger individuals
(Longhurst 2006). For example, reproductive output of a
size-selectively harvested intertidal marine limpet Cymbula
oculus, inside a marine protected area (MPA) was found
to be 80-fold higher than that of exploited populations
(Branch & Odendaal 2003). For this species, the biomass
of protected populations was also substantially higher
(30–90%) than exploited ones (Branch & Odendaal 2003),
and similar differences exist in other species such as Lottia
gigantea, an intertidal limpet from California that is
also size-selectively harvested (P.B. Fenberg and K. Roy,
unpublished). It is important to note that a reduction
in biomass by itself does not necessarily indicate that
harvesting is size selective; increased mortality due to
harvesting in general is expected to reduce the standing
biomass of exploited populations and it is now well
documented that the current biomass of many exploited
fish stocks represent a fraction of their historical levels
(e.g. Jackson et al. 2001; Myers & Worm 2003). What
separates the effects of size-selective harvesting from
overexploitation is the observation that the former has the
potential to change the fundamental scaling relationship
between size and biomass predicted by macro ecological
theory (Jennings & Blanchard 2004). When removal is size
selective, the slope of the size–biomass relationship tends
to change abruptly between size classes that are protected
from fishing and those that are not, with the former
slope being consistent with that predicted from energy-
equivalence theory (Jennings & Blanchard 2004).
Changes in sex ratio
For some species size-selective harvesting can directly or
indirectly lead to the preferential harvesting of one sex and
thus has the potential to alter the breeding sex ratio. In
the aquatic environment this is most commonly seen in
sequentially hermaphroditic fish and invertebrates, where
all individuals start out as one sex and then change to
the other as they grow older and larger. Size-selective
harvesting of these species thus preferentially removes the
larger sex and can limit the reproductive potential of the
population if it alters the sex ratios (Birkeland & Dayton
2005). As with age at maturity, life-history theory predicts
that individuals should change sex at a younger age in
response to high adult mortality and show changes in growth
rate if sex change is under exogenous control (Warner 1975;
Charnov 1979; Charnov 1981). If adult mortality rates are
unnaturally high because of size-selective harvesting it is
predicted that the age (size) at sex change will be reduced
in order to compensate for the impacts on breeding sex
ratio (Charnov 1981; Armsworth 2001; Platten et al. 2002).
Such trends are evident in some size-selectively harvested
fish (Cowen 1990; Buxton 1993; Platten et al. 2002; Hawkins
& Roberts 2004) and invertebrate species, particularly
shrimp (Charnov 1981; Hannah & Jones 1991). On the
other hand, this compensatory response may not occur if
harvesting pressure is intense enough to not allow adequate
time for sex change (Coleman et al. 1996; Hawkins &
Roberts 2004). Additionally, if size at sex change is fixed, as
it appears to be for some species (Branch & Odendaal 2003;
Munday et al. 2006), then early age (size) at sex change is
not likely under harvesting pressure. In these cases, the
loss of larger individuals leads to drastic changes in the
population sex ratio (Branch & Odendaal 2003).
Even for non-hermaphroditic aquatic species, size-selective
harvesting can disproportionately affect one sex. Many
crab and lobster fisheries preferentially harvest (directly
or indirectly) the larger males in the population, resulting
in female-biased sex ratios (Paul & Adams 1984; Wenner
1989; Smith & Jamieson 1991; Castilla et al. 1994; Sato et al.
2005; Sato & Goshima 2006). A skewed sex ratio can limit
EFFECTS OF SIZE-SELECTIVE HARVESTING 215
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Journal compilation © 2007 Blackwell Publishing Ltd
the reproductive potential of fished populations via sperm
limitation or delayed mating (Sato & Goshima 2006).
However, field observations are usually difficult and only
indirect evidence exists for the effect of male-focused crus-
tacean fisheries on reproductive success (Smith & Jamieson
1991; Carver et al. 2005; Sato et al. 2005).
In some freshwater eels, sex is environmentally deter-
mined with males differentiating at a younger age and
smaller size than females, and extreme male biases found in
harvested eel populations have been partly attributed to the
size-selective nature of the fishery (McCleave & Jellyman
2004; Beentjes et al. 2006). Population sex ratios can be more
directly impacted when harvesting is both size and sex
selective. For terrestrial species, a female-biased sex ratio
is perhaps the most common direct effect of sex-selective
hunting (Ginsberg & Milner-Gulland 1994; Milner et al.
2007). However, at least for polygynous species, a skewed
sex ratio is commonly assumed to have little negative
effect on population growth since one male can potentially
inseminate many females (Caughley 1977; Ginsberg &
Milner-Gulland 1994). In fact, recruitment rates for some
ungulate populations are resilient to unnaturally skewed
sex ratios (Milner et al. 2007) and somewhat ironically,
male-selective hunting may have actually contributed to
an increase in population growth rate and the eventual
overabundance of deer populations across much of Europe
and North America (along with other factors such as reduced
natural predation and increased food availability; Cote
et al. 2004). But of course, there is a limit to the bias in sex
ratio on the long-term viability of a population. Population
crashes attributed to reduced fecundity of sex-selectively
hunted ungulate populations have been documented for
species such as the saiga antelope (Saiga tatarica tatarica)
(Milner-Gulland et al. 2003; Milner et al. 2007).
Ecological effects
There is increasing recognition that exploitation has
ecological consequences and can lead to large changes in
community composition and the functioning of ecosystems.
Much of this work has focused on the effects of over-
exploitation of fish species (Tegner & Dayton 1999), and
it can be difficult to separate the ecological effects of over-
exploitation from that of size-selective harvesting. Moreover,
the ecological consequences of harvesting depend, at least
partly, on the functional role and competitive dominance of
the target species (Kaiser & Jennings 2001). Thus, the effects
are system specific and sometimes quite complex. In Chile,
size-selective harvesting reduces the size and abundance
of the large limpet Fissurella picta, resulting in an increased
abundance of its macro-algal food source (Moreno et al.
1984; Godoy & Moreno 1989). In areas where F. picta is
harvested (usually size-selectively), a co-occurring but
nonharvested smaller limpet Siphonaria lessoni with a
similar diet grows faster and reaches a bigger size in the
absence of large individuals of the competitively dominant
F. picta (Moreno et al. 1984; Godoy & Moreno 1989). Similarly,
increases in the abundance of a sea urchin Arbacia lixula,
has been attributed to size-selective harvesting of its
competitor Paracentrotus lividus (Guidetti et al. 2004). In
general, selective harvesting of some species can increase
the growth rate, size and abundance of other nonharvested
species because of release from competitive pressure (Godoy
& Moreno 1989; Lindberg et al. 1998; Guidetti et al. 2004).
Similar indirect effects should also be common where larger
individuals of the target species are highly territorial. For
example, a number of intertidal limpet species territorially
defend their algal grazing area by ‘bulldozing’ any intruders
such as barnacles, mussels and even other conspecifics
(Stimson 1970; Branch et al. 1992; Shanks 2002). Selective
loss of large individuals of these species can result in
community level shifts in space occupancy (Griffiths &
Branch 1997; Lindberg et al. 1998), which may be difficult to
reverse even after harvesting is relaxed. As far as indirect
effects are concerned, one of the most extreme examples
come from the Canary Islands where size-selective harvesting
of intertidal limpets may have partially contributed to
the extinction of the oystercatcher Haematopus meadewaldoi
(Hockey 1987; Branch & Moreno 1994).
Size-selective harvesting can also have an impact on
behavioural ecology. One of the better examples of this
comes from African elephants (Loxodonta africana) where
hunters preferentially kill the largest and oldest elephants
from a population for the international ivory trade (Milner-
Gulland & Mace 1991). In non-hunted populations, male
elephants typically enter a state of heightened sexual activ-
ity and aggressive behaviour known as musth between
25 and 30 years of age. At this age, they have become large
and competitive enough to win encounters with other
males (Poole 1987; Poole 1989; Slotow et al. 2000). However,
hunted populations consist of inexperienced young males
with smaller tusks. The lack of an older male hierarchy
in these populations can cause the young males to enter
musth at ages as young as 18 years old (Slotow et al. 2000).
An unexpected side-effect of their inexperience and height-
ened aggression associated with entering musth at an early
age is an increased incidence of young males attacking
and killing individuals of rhinoceros (Diceros bicornis and
Ceratotherium simum, Slotow et al. 2000; Slotow et al. 2001).
This behaviour is uncommon in populations with normal
age structures, and conservation managers have been able
to solve the problem by introducing older males to some of
the affected populations (Slotow et al. 2001).
Microevolution or phenotypic plasticity?
Size-selective harvesting clearly causes large and observable
changes in life history and ecology of exploited species,
216 P. B. FENBERG and K. ROY
© 2007 The Authors
Journal compilation © 2007 Blackwell Publishing Ltd
but whether these are evolutionary responses (i.e. have
a genetic basis) or whether they represent phenotypic
plasticity remain unclear (Law 2000). Laboratory experi-
ments have shown that in some fish size-selective harvesting
can select for genotypes with slower or faster growth
rates depending on whether large or small individuals
are selectively removed (Conover & Munch 2002). Thus,
in principle, size-selective harvesting can lead to rapid
evolutionary response and analyses of some wild populations
have found evidence for such a response (Coltman et al.
2003; Grift et al. 2003; Olsen et al. 2004). Similarly, the
failure of traits such as size at maturity to return to pre-
exploitation levels when fishing is stopped is consistent
with an evolutionary response (Conover & Munch 2002).
On the other hand, the size of many species tends to
increase once they are protected from exploitation (Halpern
& Warner 2002; Branch & Odendaal 2003; Gell & Roberts
2003; Roy et al. 2003; Hawkins & Roberts 2004), suggesting
that some of these changes reflect plasticity. In general,
for the vast majority of exploited species, the information
required to differentiate between evolutionary change and
phenotypic plasticity is currently not available (Conover
2000). In fact, even though size-selective harvesting is
widespread, we know little about the magnitude of selection
differentials due to such exploitation (Stokes & Law 2000;
Law 2001). Finally, for most exploited stocks, we have little
quantitative data on how the exploitation pressure has
varied over time. Some authors view changes in many
marine fish stocks as a relatively recent phenomenon
(i.e. latter half of the 20th century; Hutchings & Baum 2005),
while others argue that such declines extend back a couple
of centuries or longer (Jackson et al. 2001). Both views are
probably correct given that humans have been exploiting
some species of fish and invertebrates for thousands of years
(Jerardino et al. 1992; Jackson et al. 2001; Klein et al. 2004)
while other fisheries are much newer. From an evolutionary
standpoint, the lack of such information makes it difficult
to estimate how many generations have been subjected
to selective harvesting. For taxa with a short or moderate
lifespan, tens or maybe even hundreds of generations have
already been subjected to size-selective harvesting, enough
time for evolutionary responses (Conover & Munch 2002).
On the other hand, many species of marine fish and
invertebrates live for multiple decades and for these long-
lived taxa, it may be too soon to see evolutionary changes
even though they are likely in the long run (Conover 2000).
Macroevolutionary consequences of size-selective
harvesting
Size-selective harvesting is pervasive and there is no
indication that the situation is going to change in the near
future. Thus, it is reasonable to expect that such selection
pressure would lead to reduction in body sizes of many
species, especially given the high heritability of this trait
and its close relation to fitness (Law 2001). As discussed
above, such an evolutionary response has already been
documented in some species. In other cases it is likely to be
present but is yet to be detected. While virtually all of the
discussion about evolutionary response to size-selective
harvesting has focused on microevolution, size declines
due to such exploitation also have macroevolutionary
implications. In fact, body size is often thought to provide
a direct link between microevolution and macroevolution
(Jablonski 1996). The patterns and mechanisms of body
size evolution at the species level have been studied in
considerable detail and empirical data show that in many
clades average body size tends to increase over time as the
clade diversifies, a trend commonly known as Cope’s Rule
(Jablonski 1996; Alroy 1998; Hunt & Roy 2006). At present,
there are two general explanations for this trend. Stanley
(1973) suggested that most major clades tend to originate at
small sizes, and as they diversify, they add both large and
small species. However, because of a hard lower bound for
body size, the ultimate result of such passive diffusion is an
increase in both mean and variance in body size over time
(Stanley 1973; Gould 1988; Jablonski 1996). Alternatively,
Cope’s Rule can result from directional selection towards
larger body sizes (Brown & Maurer 1986; Jablonski 1996;
Hunt & Roy 2006). In either case, the natural tendency
of many clades is to add larger-bodied species over time.
Human exploitation has the potential to disrupt this
evolutionary trend by truncating the larger end of the size
distributions as body sizes of many species get smaller
because of size-selective harvesting, and many large-bodied
species face extinction because of other anthropogenic impacts
(Gaston & Blackburn 1995; McKinney 1997). Under such a
scenario, new species in the future are likely to be small since
they will be derived from small ancestors and body size is
highly heritable even at the lineage level (Smith et al. 2004).
In addition, size-selective harvesting and other human activities
counteract the selective advantages of large body size and
would thus reinforce the bias against large-bodied species.
Conclusions
Harvesting of natural resources by humans is selective by
nature (Law 2001; Longhurst 2006) and archaeological
data show that such exploitation has been going on since
the dawn of civilization (Klein et al. 2004). Size-selective
harvesting is just one example of such selective exploitation,
but because body size correlates with so many different
attributes of an organism, such exploitation has far-reaching
ecological and evolutionary consequences. Arguably, the
difference between the reductions in body size seen in
archaeological kitchen middens (Jerardino et al. 1992)
and those due to fishing over the last couple of centuries
is essentially one of scale. Today, size-selective harvesting
EFFECTS OF SIZE-SELECTIVE HARVESTING 217
© 2007 The Authors
Journal compilation © 2007 Blackwell Publishing Ltd
affects many species and exploitation pressure is higher
than ever before (Pauly et al. 1998; Baum et al. 2003; Myers
& Worm 2003; Hutchings & Baum 2005). Yet we know very
little about the evolutionary and ecological consequences
of such exploitation. The majority of the information
regarding changes in species life histories in response to
size-selective harvesting have come from a handful of com-
mercially important fish, largely from waters off developed
countries (Hutchings & Baum 2005). The combination of
taxonomic, geographical (little information exists for highly
diverse tropical areas) and habitat-related bias (Hutchings
& Baum 2005) makes it impossible to reach any general
conclusions regarding the effects of size-selective harvesting.
The situation is particularly bad for invertebrates where
many species are harvested but not only do we lack infor-
mation about their life history and ecology, but also reliable
data on patterns of exploitation. Large databases of catch-rates
and other information that permit stock assessments and
analyses of population trajectories of many commercially
important fishes (e.g. Baum et al. 2003; Myers & Worm 2003)
are virtually unknown for most marine invertebrates.
Despite the paucity of specific information for many spe-
cies, it is quite clear that size-selective harvesting is having
a negative effect on the population biology of many species
of vertebrates and invertebrates. Yet the problem is not
recognized in most management plans for fish and marine
invertebrates that still mandate a minimum size restriction
(Conover & Munch 2002). In addition, illegal size-selective
harvesting of intertidal invertebrates is a growing but
under-appreciated problem in many parts of the world and
even where regulations exist, they are rarely enforced
(Branch & Odendaal 2003; Roy et al. 2003). Despite all this,
the increases in size and biomass of exploited species within
MPAs suggest that for many species, it may not be too late
to reverse the negative ecological and evolutionary con-
sequences of size-selective harvesting (Halpern & Warner
2002; Roy et al. 2003). However, achieving that would require
us to stop preferentially removing the larger and older
individuals in a population and design harvesting strate-
gies that would preserve the size-frequency distributions
that characterize the unexploited state of a species. Sugges-
tions regarding such strategies are already available in
the literature (e.g. Conover & Munch 2002; Jennings &
Blanchard 2004; Birkeland & Dayton 2005; Hutchings
& Baum 2005). More generally, mitigating the effects of
size-selective harvesting would require us to shift from
management strategies that are designed to maximize yield
(Longhurst 2006) to those that can preserve the natural
variations that characterize species and ecosystems.
Acknowledgements
We thank Thomas Smith and Louis Bernatchez for the invitation to
participate in this summit and G. H. Engelhard and an anonymous
reviewer for insightful comments on a previous version of this
manuscript. This work was supported by a grant from NOAA
California SeaGrant (to K.R.) and an EPA STAR Fellowship (to
P.B.F.).
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Phillip Fenberg is interested in life-history evolution, conservation
biology and macroecology. Kaustuv Roy’s research interests include
macroecology, macroevolution, biological effects of climate
change and marine conservation.
... Studies have also shown that the status of fish abundance is Frontiers in Environmental Science | www.frontiersin.org June 2021 | Volume 9 | Article 663169 6 related to its reproductive rate, growth habits, seasonal differences in the living environment, and human activities (especially fishing) (Fenberg and Roy 2008;Li 2008;Rochet and Benoit 2012;Guo et al., 2018;Xu et al., 2020;Marrakchi et al., 2021). Considering the time of the investigation, the spring and summer seasons were before and after the South China Sea fishing moratorium, but summer was the most disturbed, while the fish community structure in spring was less disturbed than summer. ...
... The particle size is a prominent structural feature of fish communities and significantly reflects fish movement (Bainbridge 1958), predation (Lundvall et al., 1999;Scharf et al., 2000), reproduction (Woodward et al., 2005), the mortality rate (Peterson and Wroblewski 1984), and fishing (Fenberg and Roy 2008). The trophic level of organisms is closely related to the fish size (Jennings et al., 2001;Trebilco et al., 2013). ...
... The trophic level of organisms is closely related to the fish size (Jennings et al., 2001;Trebilco et al., 2013). Mainly, the fish grain size structure is influenced by the productivity level of the area, fishing intensity (especially overfishing), species compensation ratio (number of breeding population), and environment, all of which influence the BSS of fish (Jennings and Reynolds 2007;Fenberg and Roy 2008;. In this study, the main fish species in the smallest grain size (2-0 grain size) during spring, summer, autumn, and winter seasons were Glossogobius olivaceus, Apogon lineatus, Frontiers in Environmental Science | www.frontiersin.org ...
... On the other hand, evolutionary changes have been recorded in many species subject to selective harvest (e.g., [47][48][49]). Such a type of harvesting is not unusual in both marine and terrestrial habitats. ...
... Such a type of harvesting is not unusual in both marine and terrestrial habitats. Empirical studies are increasingly documenting the negative effect of the preferential removal of larger individuals of a species on its demography, life history, and ecology [48]. Body size is one of the most important traits of an organism correlating with many aspects of its biology. ...
... This can be followed by a reduction in survival among juveniles triggered by the growing infanticidal behavior of less dominant males [52,53]. In addition, for long-lived species with high natural survival rates of adults, size-selective hunting may cause changes in a trait such as reproductive investment [48]. Reproductive value (RV) refers to the expected individual contribution to population growth that varies with age such that the highest RV values correspond to mature adults of middle ages and the lowest ones to juveniles [54]. ...
Article
Full-text available
We examine population trends in light of male harvest data considering the long-time series of population data on northern fur seals at Tyuleniy Island. To answer the question has the way males were harvested influenced the population trajectory, we analyzed the visual harem size and birth rate dynamics of the population, as well as the strategy and intensity of the harvest. We analyzed the dynamics of the sex ratio in the early (1958–1988) period to estimate parameters in the late period (1989–2013) based on the observed number of bulls and pups, while utilizing the distribution of reproductive rates obtained from pelagic sealing. Using a matrix population model for the observed part of the population (i.e., the male population), we analyzed the population growth rate associated with changes in both birth and survival rates considering the stochastic effects. Observations allow us to reject the hypothesis of nonselective harvest. Among the variety of natural and anthropogenic factors that could contribute to the decrease in the birth rate in the population, the effect of selective harvesting seems to be the most realistic.
... Comparing the two areas (polluted and unpolluted), the animals present in the unpolluted area have greater weight and mean carapace width than animals in the polluted area. The presence of individuals with larger carapaces in the unpolluted area is evidence of the low fishing activity [22][23][24][25] and the environmental quality of the region, Regarding the posterior gills ( Figure 1B), significant HSP70 levels were found in acclimatized animals from polluted environments compared to acclimatized animals from unpolluted environments (p = 0.049). In addition, an increase in HSP70 levels was also observed in acclimatized animals from polluted environments compared to field animals (p = 0.049). ...
... Comparing the two areas (polluted and unpolluted), the animals present in the unpolluted area have greater weight and mean carapace width than animals in the polluted area. The presence of individuals with larger carapaces in the unpolluted area is evidence of the low fishing activity [22][23][24][25] and the environmental quality of the region, which is a conservation unit, considered pristine and free from environmental pollutants, mainly metals [3,4,13]. ...
Article
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This study analyzed field and acclimatized (7 days) mangrove Ucides cordatus crabs from polluted and unpolluted environments to compare their HSP70 levels. The animals were cryo-anesthetized and dissected. Gills (anterior and posterior) and hepatopancreas were collected to evaluate total proteins and HSP70 levels using ELISA (Enzyme-Linked Immunosorbent Assay) method. The acclimatized animals from polluted environments showed higher HSP70 levels in the hepatopancreas than field animals. Results showed higher HSP70 levels in laboratory animals from the polluted environment than in field animals in the posterior gills. The regulation to decrease the damage caused by the environment and the acclimatization process may not be sufficient to stabilize physiological responses, especially in animals from polluted environments.
... One of the most common harvest strategies in fisheries is size-selective harvest, where the largest individuals are targeted, generating selection on body size (Law, 2000;Stokes & Law, 2000). Size-selective harvest often results in considerable life history changes, including earlier maturation, reduction in body size, and reduced fecundity (Fenberg & Roy, 2008;Kuparinen & Merilä, 2007). Demographic processes also play a role in determining the effects of size-selective harvesting on exploited populations; processes such as density-dependence, competitive release, or Allee effects may influence how populations respond to or recover from harvest (Gobin et al., 2016;Kuparinen et al., 2014;Zipkin et al., 2008). ...
Article
Full-text available
Sustainable management of exploited populations benefits from integrating demographic and genetic considerations into assessments, as both play a role in determining harvest yields and population persistence. This is especially important in populations subject to size‐selective harvest, because size selective harvesting has the potential to result in significant demographic, life‐history, and genetic changes. We investigated harvest‐induced changes in the effective number of breeders (N̂b$$ {\hat{N}}_b $$) for introduced brook trout populations (Salvelinus fontinalis) in alpine lakes from western Canada. Three populations were subject to 3 years of size‐selective harvesting, while three control populations experienced no harvest. The N̂c$$ {\hat{N}}_c $$ decreased consistently across all harvested populations (on average 60.8%) but fluctuated in control populations. There were no consistent changes in N̂b$$ {\hat{N}}_b $$ between control or harvest populations, but one harvest population experienced a decrease in N̂b$$ {\hat{N}}_b $$ of 63.2%. The N̂b$$ {\hat{N}}_b $$/N̂c$$ {\hat{N}}_c $$ ratio increased consistently across harvest lakes; however we found no evidence of genetic compensation (where variance in reproductive success decreases at lower abundance) based on changes in family evenness (FÊ$$ \hat{FE} $$) and the number of full‐sibling families (N̂fam$$ {\hat{N}}_{fam} $$). We found no relationship between FÊ$$ \hat{FE} $$ and N̂c$$ {\hat{N}}_c $$ or between N̂fam$$ {\hat{N}}_{fam} $$/N̂c$$ {\hat{N}}_c $$ and FÊ$$ \hat{FE} $$. We posit that change in N̂b$$ {\hat{N}}_b $$ was buffered by constraints on breeding habitat prior to harvest, such that the same number of breeding sites were occupied before and after harvest. These results suggest that effective size in harvested populations may be resilient to considerable changes in Nc in the short‐term, but it is still important to monitor exploited populations to assess the risk of inbreeding and ensure their long‐term survival.
... Hence, selective harvesting with large mesh sizes would tend to select the males from the populations of these three species resulting in unbalanced sex ratios. Such scenario can alter the breeding sex ratio of an exploited population and ultimately reduce its reproductive potential (Fenberg and Roy 2008). Focusing ...
Article
Full-text available
Wetlands are among the most productive ecosystems globally characterized by dynamic interactions between terrestrial and aquatic habitats at different scales. These systems support valuable floodplain fisheries that are a major livelihood for riparian communities. Understanding the dynamics of these systems is important for developing adaptive fisheries management paradigms that will facilitate access and sustainability to this cheap but high-quality food and nutrition source. The Okavango Delta in Botswana is a large land-locked complex river-floodplain ecosystem, with a diverse biota, and high environmental heterogeneity due to periodic drying and flooding along a space and time gradient. It is characterized by a multi-species, multi-gear fishery adapted to the seasonal flood pulse. The Delta’s fish species assemblage undergoes seasonal changes driven by the flood regime. There is also a dynamic inter-annual variability in the fish species assemblage, particularly between “good” and “bad” flood years. During the wet season, high flows increase connectivity in three dimensions (longitudinal, lateral, and vertical) which facilitates dispersal of aquatic biota, nutrients, and other material among successive locations in the riverscape. However, the dry season results in alteration or reduction in aquatic habitats available for fish reproduction. Similarly, low floods may reduce inputs of nutrient resources from the terrestrial environment that support aquatic food webs and can lead to community disruption, even to the point of local extirpation of stranded fish in fragmented ephemeral pools in the floodplain. Consequently, the periodicity, magnitude and predictability of flows are the major drivers of the systems’ capacity to sustain persistent fisheries production and other ecosystem services affecting human welfare. We argue that identification of the processes that sustain production and biodiversity patterns is an essential step towards a better ecological understanding and natural resource management of river-floodplain systems. Based on this review, we debate that floodplain fisheries, like in the Okavango Delta, should be exploited using a diverse exploitation pattern to ensure a harvesting regime in balance with system productivity. Such balanced fishing pattern, based on traditional fishing practices, facilitates the provision of food and nutritional value of the fishery to marginalized communities.
... Monk et al. (2021) studied the struggle between natural and artificial selection on size and activity of Northern pike Esox lucius in a lake, showing that harvest-induced selection was stronger and yielded smaller and shyer pike; simulations revealed that the artificial selection imposed against large fish outpaced the natural selection for larger size in the lake system. Managing human harvest to mitigate selection would need to enhance harvest of lower fitness classes within populations and avoid large changes to the size-frequency distributions of a population (Fenberg & Roy, 2008). To compensate, management can shift harvesting targets for certain phenotypes with licensing or closures that disincentivize or prohibit harvest of vulnerable phenotypes that would drive selection in an unwanted direction. ...
Article
Full-text available
In nearly every ecosystem, human predators (hunters and fishers) exploit animals at extraordinarily high rates, as well as target different age classes and phenotypes, compared to other apex predators. Demographically decoupled from prey populations and technologically advanced, humans now impose widespread and significant ecological and evolutionary change. In this paper, we investigate whether there is evidence that humans provide complementary services and whether ecosystem services of predators can be maintained by humans where wild predators are lost. Our objective is to contribute to two key ecological themes: the compatibility of human harvesting within ecosystems and management approaches in consideration of the intentional or unintentional loss of predators. We reviewed evidence for five key effects of predators: natural selection of prey, disease dynamics, landscape effects, carbon cycling and human well‐being. Without carefully designed management strategies, such changes can impose harm to ecosystems and their constituents, including humankind. Ultimately, we applied this information to consider management paradigms in which humans could better support the role of, and potentially behave more like, apex predators and discuss the challenges to such coexistence. Read the free Plain Language Summary for this article on the Journal blog. Read the free Plain Language Summary for this article on the Journal blog.
... Traditional artificial selection studies focus on the impacts associated with harvesting activities, such as hunting and fishing. These activities, when performed passively (as opposed to under selective or 'active' conditions, whereby harvesters are instructed by managers to target a specific trait), typically target larger body sizes, resulting in selection for the survival of smaller individuals across both terrestrial (Stenseth & Dunlop, 2009) and aquatic environments (Fenberg & Roy, 2008). The selective removal of 'desirable' morphological traits from individual populations through these processes is well-documented (Darimont et al., 2009). ...
Article
The artificial selection of traits in wildlife populations through hunting and fishing has been well documented. However, despite their rising popularity, the role that artificial selection may play in non‐extractive wildlife activities, for example, recreational feeding activities, remains unknown. If only a subset of a population takes advantage of human‐wildlife feeding interactions, and if this results in different fitness advantages for these individuals, then artificial selection may be at work. We have tested this hypothesis using a wild fallow deer population living at the edge of a capital city as our model population. In contrast to previous assumptions on the randomness of human‐wildlife feeding interactions, we found that a limited non‐random portion of an entire population is continuously engaging with people. We found that the willingness to beg for food from humans exists on a continuum of inter‐individual repeatable behaviour; which ranges from risk‐taking individuals repeatedly seeking and obtaining food, to shyer individuals avoiding human contact and not receiving food at all, despite all individuals having received equal exposure to human presence from birth and coexisting in the same herds together. Bolder individuals obtain significantly more food directly from humans, resulting in early interception of food offerings and preventing other individuals from obtaining supplemental feeding. Those females that beg consistently also produce significantly heavier fawns (300–500 g heavier), which may provide their offspring with a survival advantage. This indicates that these interactions result in disparity in diet and nutrition across the population, impacting associated physiology and reproduction, and may result in artificial selection of the begging behavioural trait. This is the first time that this consistent variation in behaviour and its potential link to artificial selection has been identified in a wildlife population and reveals new potential effects of human‐wildlife feeding interactions in other species across both terrestrial and aquatic habitats. Human‐wildlife feeding interactions are increasingly popular, yet the role that they may play in the artificial selection of behavioural traits in wildlife populations remains unexplored. This work begins to unravel the complex dynamics and impacts of these interactions, opening up new dimensions for human‐wildlife studies.
... Over time, this selective removal of desirable phenotypic traits (large body size) lowers the occurrence of such traits in the affected population ( Branch & Moreno, 1994 ;Allendorf & Hard, 2009 ). This has the potential to lead to several negative outcomes for the harvested population including lower fitness and reduced reproductive output ( Lawton & Hughes, 1985 ;Moreno, 2001 ;Fenberg & Roy, 2008 ;Perez et al. , 2009 ). ...
Article
Size-selective harvesting of intertidal molluscs is a common practice. However, the effects of long-term traditional harvesting remain unclear. Changes in mean shell size are generally taken as evidence of changes in harvesting intensity. However, mean shell size is also influenced by environmental pressures, which may confound the analysis of size variation over time. In this study, we apply geometric morphometrics to historical data from two shell middens, in an attempt to classify Littorina littorea shells to their environmental origins, prior to carrying out shell size analysis. Using this method, shell shape was found to be consistent within and between the midden sites. Based on comparison with modern populations from shores of known wave exposure, the midden shells were found to be more consistent with sheltered shores, and to differ most from the very exposed shore sites, the latter of which are located adjacent to the midden remains. The mean shell size was significantly smaller in the more recent midden site. We hypothesize this reduction is caused by an increase in harvesting intensity over this period. It is also possible, given the very slight reduction in shell size, coupled with certain life-history traits of L. littorea, that the change in shell size was caused by slight differences in environmental conditions. The use of midden shells and morphometrics has the potential to provide an insight into previous environmental conditions and past harvesting practices, which may be used to inform current harvesting practices.
... That said, at various points, Ceramic Age peoples, and possibly the Archaic before them, did cause localized depressions and depletions of faunal (especially marine) species due to intermittent over-predation (Allendorf and Hard 2009;Carlson and Keegan 2004;Fenberg and Roy 2008;O'Dea et al. 2014;Rainey 1940;Rouse 1952;Siegel 1993;Wing and Wing 2001). There is also evidence for the translocation of mainland animals such as agouti (Dasyprocta sp.), guinea pigs (Cavia porcellus), opossum (Didelphis sp.), dogs (Canis familiaris), and deer (family Cervidae) (deFrance et al. 1996;Giovas 2017b;Giovas et al. 2011Giovas et al. , 2016Laffoon et al. 2013;LeFebvre and deFrance 2014;Newsom and Wing 2004;Stokes 1998;Wing 1989Wing , 2008Wing and Wing 1995), as well as a suite of horticultural plants and tropical root crops -some of which may have occurred in the Archaic (Newsom 1993(Newsom , 2008Pagán-Jiménez et al. 2015;Pagán-Jiménez and Carlson 2014). ...
Thesis
Full-text available
The pre-Columbian colonization of the Caribbean is traditionally described as a series of migrations from coastal South America moving northward, island to island, as “stepping-stones.” As the southernmost island in the Antilles archipelago, just 90 miles off Venezuela, the island of Grenada is assumed to be the crucial first “step” in these migrations. However, too little archaeological data was available to substantiate this claim. This dissertation project was designed to fill the gap. Using the Ideal Free Distribution (IFD), a heuristic from Human Behavioral Ecology, a predictive model was built to test areas of high-probability for early settlement on Grenada via an island-wide “radiocarbon survey” that collected artifactual, soil, and radiocarbon samples. Dated samples were refined via Bayesian methods and compared to ceramic evidence to place each site within an island-wide settlement chronology. Modern environmental data was then used to determine suitability rankings and common characteristics of settlement decisions. At present, the results confirm site locations on Grenada followed a pattern consistent with the IFD, which not only allows prediction of previously undiscovered sites but also infers subsistence practices, ecological impacts, and certain cultural values. Grenada’s settlement chronology begins with an early, Archaic Age fisher-forager presence possibly as early as 3-4000 BC, in line with the “stepping-stone” hypothesis. However, Ceramic Age settlements (which appear in Puerto Rico and the northern Lesser Antilles by 500 BC) were not established until hundreds of years later than the northernmost islands, despite their origins in South America. This corroborates an emerging hypothesis that the southern Caribbean was largely skipped by the earliest waves of Ceramic peoples, perhaps because the social milieu and domesticated landscapes of the northern islands were more attractive. Grenada's peak, pre-Columbian population occurred during a time of heightened climatic unpredictability (AD 750-900), with dramatic changes in material culture (including the appearance of rock art and new ceramic styles) that mimic similar occurrences in lowland South America. Using Resilience Theory as a guide, this research suggests the influx was likely the result of continued immigration from the mainland, probably the Guianas region (comprising modern Guyana, Suriname, French Guiana, and bordering regions of Venezuela and Brazil). This may also have been the route taken by later Cayo potters (“Island Caribs”) just prior to Spanish Contact. When the French finally settled Grenada in 1649, they reported two distinct indigenous groups—“Caraïbe” and “Galibis.” The Caraïbe were living in villages that had been continuously occupied since the earliest ceramic groups (AD 200-300), and it is argued that they were still making Suazan Troumassoid pottery. The Galibis, on the other hand, were living in sites that align with the arrival of Cayo pottery elsewhere, ~AD 1250. These sites indeed contain Cayo ceramic types. Ultimately, this dissertation lays the baseline for more intensive studies. Now that we know where 87 of the sites are, their general character, and their general chronological placement, more targeted investigations driven by more specific types of questions can be researched.
Article
Trophy hunting can affect weapon size of wild animals through both demographic and evolutionary changes. In bighorn sheep (Ovis canadensis Shaw, 1804), intense harvest of young males with fast-growing horns may have partly driven long-term decreases in horn size. These selective effects could be dampened if migrants from protected areas, not subject to artificial selection, survived and reproduced within hunted populations. Bighorn rams undertake long-distance breeding migrations in the weeks preceding the late-November rut. We analysed records of >7 800 trophy bighorn rams shot from 1974 to 2019 in Alberta, Canada, to test the hypothesis that high harvest pressure during breeding migrations was correlated with a greater decrease in horn size. We compared areas with and without a pronounced harvest peak in late October, when male breeding migrations begin. Areas without a pronounced harvest peak in late October, that likely experienced a lower harvest rate, showed a similar temporal decline in horn size, but no increase in age at harvest suggesting a possibly weaker decline in horn growth. Our study suggests that unselected immigrants from protected areas could partly buffer the effects of intense trophy hunting only if harvest pressure was reduced when breeding migrations commence.
Book
The impact of man on the biosphere is profound. Quite apart from our capacity to destroy natural ecosystems and to drive species to extinction, we mould the evolution of the survivors by the selection pressures we apply to them. This has implications for the continued health of our natural biological resources and for the way in which we seek to optimise yield from those resources. Of these biological resources, fish stocks are particularly important to mankind as a source of protein. On a global basis, fish stocks provide the major source of protein for human consumption from natural ecosystems, amounting to some seventy million tonnes in 1970. Although fisheries management has been extensively developed over the last century, it has not hitherto considered the evolutionary consequences of fishing activity. While this omission may not have been serious in the past, the ever increasing intensity of exploitation and the deteriorating health of fish stocks has generated an urgent need for a better understanding of evolution driven by harvesting and the implications of this for fish stock management. The foundations for this understanding for the most part come from recent developments in evolutionary biology and are not generally available to fisheries scientists. The purpose of this book is to provide this basis in a form that is both accessible and relevant to fisheries biology.
Book
It seems almost trite to introduce this book by saying that man has been exploiting the intertidal zone for food for a long time. Just how long nobody knows for sure but the prehistoric inhabitants of Terra Amata, on the Mediterranean coast near Nice, ate marine intertidal animals at least 300 000 years ago. Similar impressive evidence, going back to at least 100000 years, exists for prehistoric man's consumption of intertidal animals along the South African coast. However, early man's dependence on intertidal resources probably goes back much further in time. During the last 2 million or so years temperate Eurasia experienced some 20 glaciations interspersed by warm equable periods. Different modes of life were open to man in colonizing the northern temperate zone. One was to become a "big-game" hunter, specializing, for example, on mammoths, the other to exploit marine intertidal resources. Of the two, probably the shoreline offered an easier environment for an original scavenging food-gatherer.
Chapter
In the past three decades considerable research has been devoted to the profound influence of predators on intertidal and shallow-water biotic communities. Amongst other things, this has revealed that predators may act as critical or “keystone species” (Paine 1969), restricting the abundance of competitively dominant species and, thereby, preventing elimination of other species (Paine 1966, 1971, 1974; Dayton 1971; Lubchenco 1978; Peterson 1979; Lubchenco and Gaines 1981). Yet another topic is how predator-prey relationships are maintained in a relatively stable condition, i.e. what prevents the elimination of prey by their predators (Rosenzweig and MacArthur 1963). Possible options include the fact that prey may escape elimination by virtue of size, movement or periodic temporal relief (e.g. Paine et al. 1985; Hockey and Bosman 1988).