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