Harvested Fish Size

Evolution:

A Topic in Need of

Research

By Cameron Okie

Environmental Studies Senior Project 2005

ENVS 400

Lewis and Clark College

Professor and Mentor:

Jim Proctor and Paulette Bierzychudek Fish Evolution: A Topic in Need of Research

Abstract:

Most fish harvesting methods favor catching larger individual fish. This equates to a directional selective force. Evolutionary theory supports the fact that this may be the reason why a large number of the world’s harvested fish stocks are evolving towards smaller individual sizes. Smaller sizes may reduce the productivity of already troubled world fisheries. However, evidence that fish harvesting is causing these effects in wild populations through controlled experiments is far from substantial. I propose this experiment to investigate the response to conventional size-selective gillnets on wild pink salmon (Oncorhynchus gorbuscha) populations. The experiment will analyze selection gradients and differentials of three size related traits to investigate direct and indirect selection resulting from trait correlations. If individual fish size is evolving, insight from this experiment will help fisheries management adopt policies that may increase the productivity and health of harvested fish populations.

Introduction:

It has become increasingly clear that fishing has significant, even disastrous, impacts on fish populations. Fishing causes high mortality in all major fish stocks (Stokes

& Law 2000). There are a growing number of depleted fish populations worldwide, due to over-fishing (Hutchings 2004; Sinclair et al. 2002; Botsford et al. 1997). Garcia and

Moreno (2003) of the United Nations Food and Agriculture Organization (UN FAO)

Fishery Resource Division report that approximately 74% of the world’s fish stocks are being fished at levels that are near or above their maximum sustainable yield (MSY), also

1 known as “surplus production,” (Mangel & Levin 2005). Breaking their statistics down reveals that 9% are depleted to extremely low levels and 18% are far above their MSY.

One famously over-fished species is the North Atlantic cod fish (Gadus morhua)

(Hutchings 2004). Atlantic cod, found off the coasts of Labrador and Newfoundland, supported a major fishing industry for centuries, but their abundance declined 97% from

1971 to 2001 (COSEWIC 2003; Brandt 1984). Another famous fishery collapse was the

Pacific sardine population, in which fishing may have played a major role (Botsford et al.

1997).

In addition to depleting population numbers, we are also fishing down the food chain. From 1954 to 1994, the mean trophic level of harvested fish species included in

UN FAO global fisheries statistics declined, signifying a shift from large, long lived fish to smaller, shorter-lived fish, caused by unsustainable fishing (Pauly et al. 1998).

Beyond reductions in numbers and unsustainable fishing, a variety of harvested fish populations also show trends towards earlier ages of maturity and/or smaller individual size (Law 2000). McAllister et al. (1992) used data amassed by Ricker (1978) to calculate a 34 % decrease in mean body weight of harvested British Columbia pink salmon (Oncorhynchus gorbuscha) from the early 1950’s to the early 1990’s. The average age of cod populations is lower and they evolved to smaller individual sizes with slower growth rates (Olsen et al. 2005; Sinclair et al. 2002). The same is true of North

Sea plaice (Pleuronectes platessa) and European whitefish (Coregonus lavaretus)

(Heikinheimo & Mikkola 2004; Rijnsdorp 1993).

These declines in populations, trophic levels, individual sizes, and age of maturation have most often been attributed to ocean conditions, over-fishing, and inter

2 and intra species competition (Conover et al. 2005; Sinclair et al. 2002; Stokes & Law

2000; Botsford et al. 1997; McAllister et al. 1992). Another potentially important cause, specifically for reduced fish size and age of maturation, that has received little recognition, but is conjectured, are the effects of size-selective fishing (Stokes & Law

2000; Rijnsdorp 1993; Ricker 1981).

Most fishing methods favor catching larger fish, i.e. they are size-selective by removing larger fish (Hutchings 2004; Stokes & Law 2000). Preferences for larger fish arise via market prices, fishermen’s desires, regulations on size, and gear type (Ferguson,

Dana; Schones, Mike; Birkeland 2005; Sinclair et al. 2002; Law 2000; Ricker 1981).

Selecting larger fish means that smaller fish or fish that mature earlier have better chances of surviving to reproduce. If size and maturity are genetically based, individuals may inherit these traits.

Thus far, the complex fishing management entities throughout the world do not perceive harvesting as a selective force. Instead, fishing is seen as similar to mowing a lawn and waiting for it to grow back (Conover et al. 2005). Accordingly, fishing management and policy has not addressed potential evolutionary effects on harvested fish stocks (Conover & Munch 2002). Additionally, Stokes and Law (2000) found “a real reluctance” of fisheries biologists worldwide to consider evolutionary effects of fishing.

Failure to consider evolutionary effects of human caused selection has led to dramatic and rapid evolutionary repercussions in the past. Antibiotics, along with other medical advances of the 20th century, led the US Surgeon General William H. Stewart to declare in 1967 “the war against infectious diseases has been won,” (Williams 2005).

Today, however, excessive use of antibiotics has led to the evolution of more antibiotic

3 resistant and more powerful strains of disease causing organisms (Brower & Chalk

2005). For example, in the years of WWII small doses of penicillin performed miracles that today are considered to be minimal doses for minor infections (Garrett 1994).

Similarly, insects have evolved resistance to continued use of chemical pesticides (Emlen

1977). The National Research Council (2000) cites Sun et al. (1992), Schwinn and

Morton (1990), Shelton and Wyman (1991), and Forgash (1984) in saying that many agricultural pests develop resistance to pesticides in less than three years. Another example is HIV evolving resistance to drug treatment; this process sometimes only takes six months with certain drug treatments (Freeman & Herron 2004).

For a trait to evolve three premises must be fulfilled (Freeman & Herron 2004).

First, a trait must have variation. Second, trait variations must affect an individual’s fitness, the probability that an individual’s genes will be passed on to future generations.

Third, trait variation must have a heritable component. Genetics and environmental conditions can affect trait variation. Heritability is defined as the degree of trait variation that is caused by genetic rather than environmental variation. If selection is exerted on a variable and heritable trait, evolution will occur.

Wild fish trait heritabilities are difficult to estimate. Estimating heritability of a trait requires the pairing of parental and offspring trait measurements (Freeman & Herron

2004) or the values of selection intensity and response to selection from controlled experiments (Conover & Munch 2002). In the case of fish, which are migratory and reproduce or spawn with eggs and sperm in water, matching parents to their offspring is extremely difficult. Likewise, controlled experiments to estimate fish trait evolution characteristics in the wild are difficult to design and implement.

4 The difficulty in estimating heritability, which is crucial to estimate evolutionary effects of wild fish traits, may be one reason why current fish management emphasizes environmental and over-fishing effects rather than evolutionary ones for many of fisheries’ problems.

Current heritability estimates for traits such as size or growth rate are based on other animal species or studies of fish originating from artificial fish rearing in hatcheries or aquaculture facilities. Estimates suggest that fish size and growth rate is heritable

(Stokes & Law 2000; Smoker et al. 1994).

If preferential harvesting of larger fish is causing fish to evolve to smaller sizes, it may add to the lengthy list of world fisheries’ problems. Bigler et al. (1996) cite Helle

(1989) as stating that reduction in salmon size may decrease reproductive success. Egg size, which equates to available nutrients for fish larvae and dictates larval growth rate, which affects survival, is positively associated with Atlantic silverside fish size (Menidia menidia) (Conover & Munch 2002). Smaller eggs may equate to lower survival through a longer growth period to adulthood increasing the probability of mortality (Conover &

Munch 2002). Heavy uniform selection of fish size would also reduce the genetic variability for size (Conover & Munch 2005; Stokes & Law 2000; Freeman & Herron

2004), and thus compromise a population’s ability to adapt to changing circumstances

(Freeman & Herron 2004). These repercussions would result in decreased productivity in fisheries (Conover & Munch 2002), and increased vulnerability to decline and collapse from harvesting.

The evolutionary impacts of size-selective harvest are expected to be long lasting.

5 Reduced genetic variation and the lack of reverse directional selection could increase the time frame of effects. Therefore, even if harvest strategies were changed now, or stopped entirely, it is predicted that harvested fish populations would not evolve quickly back to their original state before size-selection (Conover & Munch 2005; Stokes &Law 2000).

Gillnets are an abundant size-selective harvest method (Bromaghin 2005;

Heikinheimo & Mikkola 2004). They are common for larger, more valuable fish species such as tuna, halibut, and salmon (Hovgard & Lassen 2000). These nets are stationary walls of mesh holes that wedge or tangle fish (Brandt 1984). Fish enter a mesh hole, and if they are larger than a certain size, they cannot pass completely through. Attempted reversal and struggling results in entanglement around fish gills. Therefore, a major component of gillnet size-selectivity depends on mesh size (Hovgard & Lassen 2000).

Because of this, many gillnet fisheries are managed by gillnet size, including the

Columbia River salmon gillnet fishery (ODFW, WDFW Joint Staff Report 2005).

To review, harvested fish populations evolving to younger ages of maturity and smaller individual sizes could be detrimental to already troubled fish populations.

Individual fish size, age, or diversity of the two is positively correlated with population fitness and productivity. It is predicted that harvested populations have evolved because of size-selective fishing favoring larger sizes. However, this hypothesis is not supported by substantial evidence in wild populations. The knowledge of how and why fish are evolving could bolster ecologically and commercially important declining fish populations. Using gillnets as a model size-selective fishing strategy, I propose an experiment to determine the evolutionary response of wild pink salmon to conventional size-selective harvest.

6 Pink salmon are an ideal study species. Salmon are anadromous, meaning that they are born and spawn in rivers or streams, but spend a portion of their life feeding and maturing at sea; all salmon that spawn perish soon thereafter (Romero 2003). Salmon also generally return to spawn to their rivers of birth. Pink salmon are a unique salmon species because they follow a uniform two year life cycle (Bigler et al. 1996; Ricker

1981). The uniform life span of pink salmon eliminates the possibility of changes in age of maturity or age distributions, allowing this experiment to focus on changes in size and growth rate (Ricker 1981). The return of salmon to their rivers of origin, in order to spawn, and their uniform age at death soon after spawning, in the case of pink salmon, simplifies finding salmon offspring and accounting for different generations in an experiment.

The response of an individual trait to selection depends upon the selection intensity and heritability of a trait (Freeman & Herron 2004). However, selection acts on many traits simultaneously through direct selection or physiological and/or genetic correlations with directly selected traits (Lande & Arnold 1983). The response to this size-selective gillnet harvest experiment will determine which, of three size relevant traits, are directly selected and which may be changing due to “hitchhiking” through correlations to the trait directly under selection.

Girth, maximum fish body diameter, will be studied because I hypothesize that it is directly selected by gillnets. Length and weight will also be studied because they are particularly important for human harvest and are positively correlated in some salmonids

(Gjerden & Ghedrem 1984).

7 I hypothesize that gillnets will cause evolution of fish size because gillnets are very size-selective and wild fish size is likely heritable. I expect reductions in fish size because conventional gillnet fishing preferentially catches larger individual fish.

This proposed experiment will explore the unknown response to prevailing large size-selective harvesting, using gillnets, of wild fish populations. Multiple size-related trait response to gillnet selection analysis will reveal direct and correlated trait responses.

This will shed light upon how and why fish populations respond to size-selective gillnet harvest. Since evolution of harvested fish is currently not addressed by management and policy throughout the world, concrete evidence of the response to large size-selection could bring critical issues about gillnets and other size selective gear to policy and management’s attention. If response to large size-selection causes fish to evolve to smaller sizes, policy and management must address this in order to help sustain world fish populations.

Background:

Pink salmon populations will be harvested with a common relatively size- selective gillnet for pink salmon. A relatively non-size selective seine net fishing method will act as the gillnet’s control to investigate size-selective fishing effects on pink salmon size. The experiment will collect data from four river locations over a duration of ten years. Distinct pink salmon populations will be consistently fished with the same fishing method throughout the experiment to investigate long term fish response to size selective harvest.

Pink Salmon

8 Pink salmon are found in the eastern and western northern Pacific Ocean, as far north as the arctic and Alaska and as far south as Southern California (Romero 2003).

Upstream river migration and spawning occurs between June and late September, depending upon location (ADFG 1985). This migration period is when the experiment data will be collected.

This study can be conducted continuously for 10 years on pink salmon populations as they migrate up rivers or streams. Rivers can support two pink salmon populations, commonly termed even and odd year populations (Ricker 1981). Annually one population migrates upstream because pink salmon have a uniform life span/cycle of two years (Bigler et al. 1996; Ricker 1981).

One problem with pink salmon as a species of study for this experiment is their weaker homing behavior to find their rivers of origin as compared to other salmon, also known as wandering (Romero 2003; ADFG 1985). Pink salmon wandering will affect collected data by including data from fish that did not experience desired experimental treatments. However, only a slight degree of pink salmon wandering is documented to exist and its responsibility for error in data will be acknowledged (ADFG 1985).

Gill and Seine Nets

A seine net is a circular net wall that restricts fish horizontally and vertically within it; by pulling the circle of the seine net in tighter fish can be concentrated together

(Brandt 1984). Seine nets are a relatively non size-selective fishing method because their small net mesh size retains all adult pink salmon independent of size (Ricker 1981). The seine net contrasts greatly with gillnets because gillnets catch fish by having them swim

9 into mesh holes that entangle them around their gills. Seine nets do not entangle, but restrain fish.

Both methods of gillnet and seine net fishing are not perfectly size and non-size- selective, respectfully. Gillnets involve variation to specific size-selectivity because of the tangling of fish outside the optimum size range (Bromaghin 2005; Millar & Fryer

1999). Regarding seine nets, certain sized fish may be better at jumping over the net at the surface and fish caught may have been previously segregated by size through schooling (Schones, Mark; Ricker 1981).

Measures will be taken to reduce tangling in gillnets and escapement in seine nets.

Gillnets will not be hung loosely in the water to reduce fish becoming tangled (Oatman,

John). Seine nets will have large surface buoys which will raise the seine net above the water surface to reduce the escapement of jumping fish (Brandt 1984). Both the error from an undesired degree of non-size-selectivity in gillnets and a degree of size- selectivity of seine nets will be taken into account in this experiment.

Study Locations:

River locations may be ideal in remote areas of British Columbia and Alaska because some rivers there have large, wild (non-hatchery or aquaculture raised), and relatively un-harvested pink salmon populations of both even and odd years (McAllister et al. 1992). Populations must be large so that the fishing experiment will not threaten population survival. Local harvesting on selected experimental river locations will be avoided because it could impede the carrying out of the experiment. Fishermen may find an experiment to be a burden to their need to harvest the fish of study.

10 Since pink salmon may wander I will attempt to minimize the catching of salmon not of a river’s population. I will use rivers that are geographically isolated to decrease the chance of catching more than one pink salmon population’s individuals.

River mouths are areas where different pink salmon populations may temporarily mix (ADFG 1985). I will collect data as far upstream of the river mouth as possible.

How far upstream data can be collected will be determined by spawning ground location.

All fish swim up rivers to spawning grounds, but some may swim further than others.

Fish that spawn in certain spawning grounds may differ in any one of the size-related traits being recorded for this experiment. To catch un-biased samples of fish, based on spawning ground location, data must be collected in areas covering all spawning grounds.

To determine spawning ground locations, rivers will be surveyed during the spawning period of late June to September. Both even and odd year population spawning grounds will be surveyed because even and odd year pink salmon populations may spawn in different river areas (ADFG 1985).

For on-river data collection locations, I will choose areas of rivers that are narrow and that have few river-bottom obstructions. Bottom obstructions can tangle and damage fishing gear and not allow nets to catch fish (Soderstrom, Gary). Narrow segments of the river will be beneficial in concentrating pink salmon migrating upstream. The concentrating of fish numbers may decrease the amount of treatment implementations necessary to collect desired sample sizes, which will save time and energy.

Data Collection:

11 Odd or even year pink salmon populations of the same river will experience one of two fishing treatments (Figure 1). For each treatment, every net will catch a sample size of at least 200 fish.

Individual fish length, weight, girth (maximum diameter), and sex will be recorded for individual fish of every net’s catch in both experimental treatments. Fish weight will be recorded in grams. Fish length will be measured as the distance between the middle of the eye to the center of the fork of the tail in millimeters because morphological changes due to sexual maturation may distort length during upriver migration periods (Smoker et al. 1994). Length will be measured while fish are lying down parallel to a flat surface. Girth will also be recorded in millimeters by wrapping a tape measure around the fish. Sex will be determined by the presence or absence of a male characteristic dorsal hump, developed during maturation and spawning (Romero

2003). However, if sexual identification is not obvious through the presence or absence of a dorsal hump, internal inspection will ensure sexual identification.

A fish counter or method to estimate fish passage numbers before and after fishing is necessary. Not only will this counter determine when strong runs of salmon are occurring so that the experiment can be conducted at these times, but it will also provide the percentage of a population to experience size selective fishing and necessary numbers for the construction of selection gradients.

12 1 2

1 2

Figure 1: The two experimental treatments. The gillnet treatment arrangement involves three fishing locations. The downstream, first seine net will yield fish trait data before size selection. The gill net will yield selected fish trait data. The upstream, second seine net will yield fish trait data after selection. The seine net treatment will yield fish trait data in two locations without experimental selection. The fish counter will estimate fish population numbers that migrate upstream. Gillnet net treatments will require three fishing vessels, two seine nets, and one gillnet. Seine net treatments will need two fishing vessels and two seine nets.

Sizes of odd and even year pink salmon populations may vary in number (ADFG

1985). If a river’s odd and even year pink salmon populations are not the same size, I

will conduct the gillnet treatment on the larger population because the selective gillnet

treatment may have larger, if any, effects in smaller populations.

The number of seine or gillnet treatments a pink salmon population will

experience will depend upon salmon migration time periods determined by preliminary

observational studies. The treatments will be temporally spaced equally throughout the

migration time period to eliminate any bias of fish data regarding time of upstream

13 migration. Smoker et al. (1994) found a non-uniform relationship between pink salmon size and time of migration.

Fishing treatments will be conducted at dawn and dusk, during high water levels, and during incoming tides, when more individuals will migrate upstream (Romero 2003;

Kingsbury 1994; Cheng 1991). The fish counter will also indicate when individuals are migrating upstream in high numbers. High numbers of upstream migrating salmon will help to ensure that all net sample sizes are met.

Seine net caught fish will be sampled using a smaller net able to dip into the restricting seine net. The dip net will be lowered to different depths and areas within the seine to control for bias of certain fish located in different areas of the seine net catch.

The number of smaller samples will continue until the sample size is reached. Smaller catches may use the entire seine catch or additional seine catches as needed to meet the

200 fish individual sample size. Large catches will release fish in the seine once the sample size is achieved. This assumes that containment in a seine net will not affect salmon fitness. I assume this because seine nets have small mesh sizes, which decrease the likely hood of tangling, which may harm salmon. In addition, seine nets will not be tightly restricted around caught pink salmon. Tight net restrictions may also harm salmon.

The gillnet catch will use the method of recording for every nth fish of the total catch for recording trait data. The nth fish to be taken and recorded will depend upon the total estimated catch size to achieve the desired sample size. If catch size is low then the entire catch may be recorded. No fish caught in gillnets will be returned to spawn.

14 I will conduct gillnet and seine net treatments using the spatial and temporal

design created by McAllister et al. (1992) (Figure 2). This experimental design has both

temporal and location replicates of gillnet and seine net treatments. The replicates allow

for control of either environmental variation or gillnet treatment effects. For example, in

year one the effects of gillnet and seine net treatments can be compared between different

rivers, assuming environmental variation in locations was similar. In river location one

gillnet and seine net treatments effects can be compared between years. Comparison of

treatments on the same river may have reduced environmental variation in years because

of the same river location.

Temporal Design

Year 1 2 3 4 5 6 7 8 9 10 1

2

River 3

Location 4

Key Gillnet Treatment

Seine Net Treatment

Figure 2: The temporal design of the experiment’s two treatments on four river locations. Each river’s two pink salmon populations will experience only one of two treatments over the ten year experiment. Because pink salmon have a two year life span and return to river’s of birth to spawn gillnet and seine net treatments will alternate each year for a given river location.

15

Having equal fish mortality, measured by individual fish numbers, between treatments, river locations, and years would be ideal. The only difference desired between treatments is the presence or absence of gillnet fishing. However this is a nearly impossible task. Predicting whether gillnets catch large or small numbers of fish in a given year is very difficult. Once a sample size of at least 200 fish is achieved with a gillnet, releasing fish that are extras, as with the seine nets, is decreasing the selection intensity of gillnets in the experiment and would be difficult. Fish caught in gillnets are usually entangled around their gills (Brandt 1984), which are vital breathing organs (Hale

2003). This entangling most likely harms fish and releasing them would probably not ensure survival or their respective fitness.

To attempt to equalize fish mortality numbers between rivers, years, and treatments, gillnets will be put into the water for limited amounts of time based on fish counter migration numbers. Also, releasing extra or catching more fish in seine nets could help equalize fish mortality numbers. More than likely, fish mortality numbers will not be consistent throughout the experiment. This will have to be accounted for in the analysis of the data.

Data Analysis:

All data will be analyzed in two forms, as collected and in standardized form.

Subtracting the mean from each trait value and dividing by the distribution’s standard deviation yields standardized values and standardized distributions (Hair et al. 1998).

16 For all fish trait analysis, male and females will be analyzed separately because male pink salmon have higher genetic variation in size (Smoker et al. 1994; Beacham &

Murray 1985). Because of this, females and males may have different responses to selection. Combining sexes in analysis would assume an equal response.

Multi-trait Selection Gradient:

I will quantify several selection gradients that graph relative fitness and three traits as described by Lande and Arnold (1983) for both seine and gillnet treatments.

Relative fitness equals an individual’s absolute fitness divided by the population’s mean absolute fitness. Absolute fitness is a number between zero and one which is based on a fitness indicator (Freeman & Herron 2004).

The absolute fitness value of zero will be assigned to gillnet caught fish. This value will be used because these fish will be caught and not released to reproduce. Fish which are not caught and spawn have varying degrees of absolute fitness depending on numerous traits. An ideal indicator of absolute fitness for a parent fish would be the knowledge of how many of their offspring live to reproduce. However, this method is nearly impossible in this study because of salmon’s egg fertilization strategy in open water and the fact that they are highly migratory (Romero 2003).

The best fitness measure for a selection gradient is one based on the biology of the experiment (Brodie et al.1995). Since this study is of size response to selection, a logical fitness indicator is size. Wertheimer et al. (2004) cite Foerster and Pritchard

(1941) in saying that female pink salmon size has a positive correlation between egg size and fecundity. This is supported by both the analysis of Smoker et al. (1994) and evolutionary theory (Roff 1992). However, male pink salmon size varies more than in

17 females, which may serve to conserve population fitness (Smoker et al 1994; Beacham &

Murray 1985). Based on this information, absolute fitness for female pink salmon can be based on larger size and male absolute fitness can be based on sampled male size distributions. I will use the size trait of length as the absolute fitness indicator because research and evolutionary theory do not specify a particular size trait related to fitness.

It can be assumed that natural un-fished populations represent optimal pink salmon sizes (Ricker 1978). Therefore, when the experiment begins, the recorded length distribution of males and females at seine net one of years one and two for even and odd year populations, respectively, will serve as the male and female absolute fitness baselines for each river. The male median length value will have a relative fitness of one.

Other male fish sizes will have lesser values of absolute fitness, less than one, depending on the male length distribution’s spread. The female length distributions largest length will correspond to an absolute fitness of one and shorter lengths will be assigned an absolute fitness less than one depending on the female length distribution spread. This absolute fitness assignment scheme based on seine net one catch for odd and even year pink salmon populations will be used for each river, and the ten year duration of the experiment.

The number of fish that pass and spawn will be recorded by the fish counter in a chronological fashion. This will allow the pairing of uncaught passing fish numbers to seine net one sampling trait distributions and sex numbers from the same time period.

This is necessary because Smoker et al. (1994) documented a non-uniform relationship between pink salmon time of river migration and spawning to both length and weight.

Times when uncaught fish are passing between selective fishing periods will be divided

18 in half. The first time period will be paired to the trait distributions of seine net one recorded previously; the second time period will be paired to the next seine net one trait distributions to be recorded.

Seine net two caught fish traits and fitness will be graphed into the selection gradients. Seine net one caught fish will not be graphed in the selection gradient during only gill net treatment years because these fish did not experience selection. I will assume this sample is a random sample and therefore will not affect trait distributions of gillnet or seine net two caught fish.

In total, forty eight selection gradients will be quantified. Forty for each year of the four rivers’ two populations during the 10 year experiment, in addition to eight temporally cumulative selection gradients quantifying all ten years of data for the four river’s two fish populations. Comparison will be made between individual and cumulative selection gradients for gillnet and seine net years.

Selection Differentials:

Roff (1997) defines the selection differential as the difference between the mean trait value of the population and the mean trait value of the population from which the reproducing parents were collected. The subtraction of the trait mean of seine net one caught fish (total population sample) from the same year’s trait mean of seine net two caught fish (spawner sample) equals the selection differential. The selection differential values will be calculated in all gillnet and seine net years for girth, length, and weight.

All selection differentials will be tested for statistical significance by comparing the trait distributions of seine one and seine two, for both treatments, with paired t-tests.

This equates to 40 selection differentials (Figure 1). Paired t-tests can be used because

19 the two fish trait distributions compared will be from the same fish population. Size trait data should be normally distributed to validate the paired t-test assumption of normally distributed data. Girth, length, and weight are quantitative not qualitative traits and should follow normal distributions (Freeman and Herron 2004). Sampling of fish in seine net one and seine net two will validate this assumption. Large sample sizes of 200 will make the tests robust.

The paired t-test hypotheses apply to both gillnet and seine net years. The null hypothesis, H0, is that the selection differential will equal zero. The alternative hypothesis, HA, is that the selection differential will not equal zero.

Selection Differentials and Partial Regression Coefficients:

It is important to compare the selection differentials and selection gradient partial regression coefficients to reveal which traits are truly selected. Lande and Arnold (1983) showed in their analysis of selection differentials and partial regression coefficients that selection differentials alone can yield misleading direct selective forces on traits.

Selection differentials quantitatively show how a population trait changes over time.

Partial regression coefficients quantitatively show which traits are selected and how

(positively or negatively). Comparison of the selection differentials and partial regression coefficients can assess indirect selection, through trait correlation, and direct selection. Conclusions will be based upon the statistical significance and direction

(positive or negative) of coefficients and differentials.

Gillnet, Seine Net Selection Differential Comparison:

Selection differentials for girth, weight, and length will be compared between single rivers and among rivers through two year time periods. Two year time periods

20 include both gillnet and seine net treatments and control for the number of fishing events

a salmon population experiences. For example, the selection differentials of years one

and two, three and four, five and six, seven and eight, and years nine and ten will be

compared for the four rivers (Figure 4).

Selection differential values during consecutive gillnet and seine net years on the

same river will be compared by the degree of statistical significance and value of

selection differentials. Comparing identical rivers should reduce environmental variation

in comparisons.

Comparison Comparison Comparison Comparison Comparison 1 2 3 4 5 Seine Seine Seine Seine Seine River Gillnet Net Gillnet Net Gillnet Net Gillnet Net Gillnet Net

1 yr 1 yr 2 yr 3 yr 4 yr 5 yr 6 yr 7 yr 8 yr 9 yr 10 2 yr 2 yr 1 yr 4 yr 3 yr 6 yr 5 yr 8 yr 7 yr 10 yr 9

3 yr 1 yr 2 yr 3 yr 4 yr 5 yr 6 yr 7 yr 8 yr 9 yr 10 4 yr 2 yr 1 yr 4 yr 3 yr 6 yr 5 yr 8 yr 7 yr 10 yr 9

Figure 4: The Comparison of selection differential values between temporally consecutive gillnet and siene net treatments on all rivers. Each comparison consists of one gillnet and one seine net response to selection value for each of four rivers. Selection Differential values are written as the years (yr) from which they were calculated. Environmental variation between differential comparisons is reduced through temporal proximity.

Five Selection differential values’ comparison during consecutive gillnet and

seine net years, among the rivers will also be compared with a Mann Whitney U-Test,

also known as the Rank Sum Test (Figure 4). This statistical test contrasts numerical,

unpaired samples for statistical significance by comparing the selection differential value

sums of each treatment (Walpole et al. 2002). These comparisons are unpaired because

selection differentials are compared between consecutive years. This statistical test will

yield statistical significant differences between selection differentials of gillnet and seine

21 net treatment. The sample size of eight selection differentials, four of each treatment, for each comparison is small. Unless data are very conclusive, this test may not yield statistically significant results.

Response to Selection:

Freeman and Herron (2004) define the trait’s response to selection as the difference between the entire offspring population trait mean and the trait mean of offspring from selected parents. This method to calculate the response to selection is not possible in this experiment. If the entire offspring population were permitted to spawn, differentiating between selected and non-selected parents’ offspring would be nearly impossible.

Since the gillnet or seine net year treatment is the only treatment acting on pink salmon populations, it can be assumed that any change in a trait mean or distribution can be attributed to gillnet selection. Environmental variation will likely affect generational change. However, I will consider environmental variation as a source of error in assuming that any trait change through generations is due to gillnet or seine net treatment.

The response to selection values of girth, weight, and length will be calculated for each river’s odd and even year pink salmon populations. Response to selection will equal the difference between the seine net one trait mean of year x and the seine net one trait mean of year x + 2, because pink salmon have a two year life cycle. This equates to subtracting offspring mean trait values from parental mean trait values. Each river will yield eight response to selection values and 32 total response to selection values for all four rivers.

22 Male and Female girth, length, and weight responses to selection will be calculated separately. Smoker et al. (1994) found that male pink salmon have significantly higher genetic variation for length than do females. Accordingly, male pink salmon may have a greater probability of size-related traits evolving than females.

All response to selection values will be tested for statistical significance by comparing the trait distributions of seine net one of year x with the trait distribution of seine net one of year x + 2 with unpaired t-tests. Unpaired t-tests will be used because the distributions are temporally separated by two years. During the course of two years, environmental variation may be large. This may discount the fact that gillnet or seine net treatment is the only factor creating difference between the two distributions, however replicates should help demonstrate the amount of environmental variation in comparisons.

Validation of the un-paired t-test assumption of normally distributed and large data sets should occur. Sample sizes will be at least 200 and weight, girth, and length are quantitative traits that are probably normally distributed (Freeman & Herron 2004).

The unpaired t-test hypotheses apply to both gillnet and seine net years. The null hypothesis, H0, is that the response to selection will equal zero. The alternative hypothesis, HA, is that the response to selection will not equal zero.

Gillnet, Seine Net Response to Selection Comparisons:

The comparison of response to selection in single rivers and among multiple rivers will be similar to the two methods used to compare selection differentials. One will be the Mann Whitney U-Test. The second will be comparing individual response to

23 selection values and their statistical significance. These comparisons will show

differences in response to selection between gillnet and seine net years among different

rivers and between the same rivers.

In comparisons, I will control the number of experienced gillnet and seine net

treatments. For example, the gillnet response to selection measured from year x and

year x + 2 will be compared with the seine net response to selection of year x +1 and x

+3. Both of these response to selection values have experienced the same number of

gillnet and seine net treatments. This comparison is also between the temporally closest

treatments.

Comparison 1 Comparison 2 Comparison 3 Comparison 4 Seine Seine Seine Seine Gillnet Gillnet Gillnet Gillnet River Net Net Net Net 1 yr 1,3 yr 2,4 yr 3,5 yr 4,6 yr 5,7 yr 6,8 yr 7,9 yr 8,10 2 yr 2,4 yr 1,3 yr 4,6 yr 3,5 yr 6,8 yr 5,7 yr 8,10 yr 7,9 3 yr 1,3 yr 2,4 yr 3,5 yr 4,6 yr 5,7 yr 6,8 yr 7,9 yr 8,10 4 yr 2,4 yr 1,3 yr 4,6 yr 3,5 yr 6,8 yr 5,7 yr 8,10 yr 7,9

Figure 5: The Comparison of response to selection values between temporally consecutive gillnet and seine net treatments on all rivers. Each comparison consists of four gillnet and four seine net response to selection values for the four river locations. Response to selection values are written as the two years (yr) from which they were calculated. Environmental variation between response comparisons is reduced through temporal proximity.

The Mann Whitney U-Test will test the statistical difference between four gillnet

and four seine net response to selection values for each comparison set (1-4) (Figure 5).

The four rivers’ two response values, 8 values one from each treatment, will compromise

each test’s sample size. This is not a large sample size, and the test will probably yield

non-significant results unless specific treatment data are roughly uniform in value and

sign. I wish to take a precautionary approach to statistically test these data because of

24 fish size-evolution to smaller sizes’ potential, major repercussions such as: reduced fecundity, population fitness, and increased vulnerability to decline or population collapse from harvesting. Therefore I wish to use a high alpha or significance level of .2 for the Mann Whitney U-Test.

Significance of Study: The state of the world’s fisheries is dismal. Harvested fish populations are declining, collapsing, and evolving in historically undocumented ways. International organizations and nations must re-address the status of world fisheries. In order to do so biologists and researchers need to collect related biological data for purposes of sustaining fish.

This experiment will collect meaningful knowledge of the unknown response to common large size-selective harvesting of wild fish populations. The experiment’s multi-size related trait analysis will investigate exactly how fish respond to this selection.

This information is relevant on a global scale because most fishing methods select for larger sizes (Stokes & Law 200), and will provide a sturdy foundation to further research into fish population response to harvesting.

Current policy regulates fishing gear, fishing times, and fishing’s catch size to conserve population numbers. If management knew the effects of common larger size- selective harvesting, the potential evolution of fish to smaller sizes could be halted or reversed. The probabilities of susceptibility to population decline and collapse from harvesting could be lowered through conservation or enhancement of fish genetic variability, fecundity, productivity, harvestable biomass, and adult survival.

Regulations on harvest methods or fish size based on knowledge of how fish

25 respond to size-selective harvest could increase population fitness and therefore create more robust fisheries. Ocean, river, and lake ecosystems would benefit from healthier fisheries. More robust fish populations would also be more likely to sustain income and nourishment related fish harvest for future generations.

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