Significant Effects of Fishing Gear Selectivity on Fish Life History

LIANG Zhenlin, SUN Peng YAN Wei HUANG Liuyi and TANG Yanli

1) College of Fisheries, Ocean University of China, Qingdao 266003, P. R. China

2) Marine College, Shandong University, Weihai 264209, P. R. China

Significant Effects of Fishing Gear Selectivity on Fish Life History

LIANG Zhenlin1),2),*, SUN Peng1), YAN Wei1), HUANG Liuyi1), and TANG Yanli1)

1) College of Fisheries, Ocean University of China, Qingdao 266003, P. R. China

2) Marine College, Shandong University, Weihai 264209, P. R. China

Over the past few decades, extreme changes have occurred in the characters of exploited fish populations. The majority of these changes have affected the growth traits of fish life history, which include a smaller size-at-age, an earlier age-at-maturation and among others. Currently, the causes of these life history traits changes still require systematic analyses and empirical studies. The explanations that have been cited are merely expressed in terms of fish phenotypic adaptation. It has been claimed that the original traits of fish can be recovered once the intensity of exploitation of the fish is controlled. Sustained environmental and fishing pressure will change the life history traits of most fish species, so the fish individual’s traits are still in small size-at-age and at earlier age-at-maturation in exploited fish populations. In this paper, we expressed our view of points that fishing gear has imposed selectivity on fish populations and individuals as various other environmental factors have done and such changes are unrecoverable. According to the existing tend of exploited fish individual’s life history traits, we suggested further researches in this field and provided better methods of fishery management and thereby fishery resources protection than those available early.

size composition; life history; fishing gear selectivity; fishery management

1 Introduction

Trait changes of exploited fish populations are receiving more and more attentions (Zhanget al., 2010a). The principal changes in fish history include early maturation, shift to earlier age structures and miniaturization (Stergiou, 2002). Fish miniaturization, the most obvious and striking characteristic, includes the miniaturization of fish community, population structure and fish individuals. Fish community miniaturization is a change in dominant population in a certain area. For example, when fisheries experts surveyed the fishery resource in Taihu area, they found that the large-scale community structure of economically important fish, such as black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idellus), silver carp (Hypophthalmichthys molitrix), and Bighead carp (Hypophthalmichthys nobilis) in family Cyprinidae, had been replaced by small fish with low economic values (Zhu, 1979). The miniaturization of population structure refers to the increase in the number of younger fish and the decrease in the number of older fish. For example, research on Black scraper (Thamnaconus septentrionalis) in family Aluteridae showed a tendency toward an increase in the number of juveniles and miniaturization (Zhan, 1995). The miniaturization of individuals means that the body length and weight of a fish species at the same age decrease consistently over time (Linet al., 2010).

Primarily, this paper focuses on the miniaturization of individuals. In recent years, many scientists have investigated the causes of fish miniaturization and life-history traits (Vainikkaet al., 2010). The subjects of these investigations include the growth traits of Atlantic silversides (Menidia menidia) in laboratory (Conover and Munch, 2002; Conoveret al., 2005), evolution induced by fisheries (Law, 2007) or harvesting of other wild animals (Allendorf and Hard 2009; Allendorfet al., 2008), life history changes induced by size-selective fishing gear in Northeast Arctic cod (Gadus morhua) (Jorgensenet al., 2009), the effects of overexploitation on population recovery (Hutchings, 2005) and the application of numerical analyses in exploited populations (Zhuet al., 2005). These investigations aimed at developing theoretical explanations to the causes and mechanisms of changes in characters in fish populations.

The causes of changes in the traits of fish life history include environmental and fishing factors (Law, 2000; Safina, 2012). Environmental pressures, such as habitat destruction, interference with migration, decreases in spawning sites (Caoet al., 1991), food shortages (Heathet al., 1996) and climate change (Daufresneet al., 2009), inhibit the growth of fish and result in the miniaturization of a fish population. In addition, increasing evidences have demonstrated that changes in fish population structure in response to fishing pressures cannot be ignored (Law, 2000). Therefore, research on fishing effects in thecontext of a changing fishing gear selectivity parameters is important for fisheries management (Sun, 2013). This view is an essential component of ecological fishing gears to fisheries management.

Current status of the changes is seeable in the biological characters of two commercially exploited fishery populations in China, small yellow croaker (Larimichthys polyactis) and hairtail (Trichiurus japonicus). These two fish species are commercially important in Yellow Sea and East China Sea (Figs.1 and 2). Studies have indicated that some of their life history traits have clearly changed as their populations were continuously overexploited in recent decades. These changes are primarily reflected in the miniaturization of individuals and the sexual maturation at earlier ages (Renet al., 2001; Linet al., 2010; Zhanget al., 2010b). Consequently, the proportion of small-sized individuals in a population or small-sized fish species in a fish community is increasing as overfishing goes on (Wu and Wang, 1991).

Fig.1 The anal length distribution of hairtail (Trichiurus japonicus) at different ages as was observed in East China Sea (Zhang, 2007).

Because comparison of size in harvested populations involve fish individuals at the same age, those in small sizes at a given age in a fish population can be attributed to a slow growth rate which results from the selection of genes (Law, 2002). We concerned this because this type of slow growths produces a number of defects. Walshet al. (2006) conducted a research on the fifth and the sixth generation of an exploited fish population. They found that fish in large-harvested population did not show a willingness to obtain food or defend themselves against predators. The offspring of these generations of fish became smaller in size and number and less healthy than their parental generation (Conover, 2007). Experimental research showed that changes in the traits of fish can result not only from the effects of various environmental factors but also from long-term selective fishing. Furthermore, because of the deleterious effects of fishing on the presence of individual genes, such effects may also involve genetic factors (Lianget al., 2008).

2 Fishing Gear Selectivity Affects Fish Morphological Traits in Fish History

Fishing gears impose various forms of selection on fish. The main trait targeted by such a selection is the body length of fish (Tanget al., 2010). Fish size is of primary interest to fishermen and lager fishes are often of higher value. Larger size and older fishes can also contribute disproportionately to productivity through greater weightspecific fertility and larval quality (Allendorf and Hard, 2009), in recent years, many researches on the changes of fish life history because of overfishing and fishing gear selectivity (Huanget al., 2005; Linet al., 2010). Most of these researches are based on the data of fishery survey recourses, at the same time, many simulation methodology was used to model fishing induce the changes of fish life history (Wang and Thomas, 2009; Lianget al., 2008). The purposes of these analyses were to investigate the effects of trawl and gillnet selectivity and fishing intensity on body length in simulated fish population and to discuss the stability of such influences. The results of the experiments performed with simulation showed that long-term fishing stress has a substantial effect on the change in fish body length (Lianget al., 2008). Moreover, the body length cannot return to the original. The change was unidirectional and irreversible. So long-term fishing gear selectivity can induce the changes of fish life history, which include body size miniaturization and the miniaturization can not recover (Lianget al., 2012).

2.1 Trawl Selectivity Affects Fish Morphological Traits

Most kinds of fishing gear do not catch all fish with equal probability but are selective for certain types of fish in one way or another. Often, this selectivity is based on body size. For example, small fish may slip through gill-nets and large fish avoid getting caught, while fish with a girth close to the mesh size are most effectivelyharvested (Jorgensenet al., 2009). In the experiment of Atlantic silversides Menidia menidia(Conover and Munch, 2002), harvesting of the largest individuals, which is analogous to trawl fishing, led to heritable changes towards smaller fish, slower growth and so on. Because these changes effects caused by trawling-like size-selectivity, the selectivity of fishing gear is measured by referring to a selectivity curve. The selectivity curve of trawl gear resembles a ‘S’ curve. The gear harvests large fish and bypasses small ones (Sun, 2004). The simulation experiments use a selectivity curve to determine the effects of long-term fishing with trawl gear with particular selective parameters on a fish population with a specific value of body length. The simulation is used to study whether the influence of fishing is reversible after the fishing has been discontinued. The results of the simulation offer a complete explanation of the distinctive effect of fishing stress on the traits of fish. This effect is especially marked after 10 years of fishing. Fishing plays a major role in the changes in fish morphological traits size composition (Lianget al., 2008) and the mean body length cannot return to its initial value in the population before the beginning of fishing (Fig.3).

Fig.3 The change in average length at different exploited rates.

2.2 The Selective Influence of Gillnet on the Individuals’ Miniaturization

In addition to the simulation approaches that were used to study the influence of trawl gear on the individuals’miniaturization of fish, statistical simulations of the selective effects of gillnet were performed (Lianget al., 2012). Some research suggested that the bell-shaped selectivity curves of gillnets may be better from a sustainability perspective (Law and Rowell, 1993; Law, 2007). Long-term fishing with gillnets profoundly influenced the harvested fish population. First, overexploitation has an irreversible influence on the abundance and the characters of a fish population and ultimately causes the exhaustion of the resource if it is not halted. Second, under an appropriate and constant rate of exploitation, the effects of gillnet fishing on the body length composition in a fish population were largely determined by the selection of approximately optimal body length, the optimal body length for gillnet fishing. Therefore, the choice of an appropriate optimal body length for gillnets fishing can achieve an ideal selective effect and prevent the eventually apparent miniaturization of individual fish. This approach can have a positive effect on both the normal propagation of fish populations and the sustainable exploitation of fishery resources. Many researches give the evidences that fishing with gillnets is better able to withstand life-history evolution, and maintains yield over a wider range of fishing intensities (Jorgensenet al., 2009; Huseet al., 2000; Hilborn and Minte-Vera, 2008).

Gillnets and trawls have different selective effects, although both of them potentially have irreversible influences on the body length composition of a fish population (Sun, 2004). Gillnet is a passive fishing device, which shows selective effects at intervals on the traits of a fish population. This gear is only suitable for catching fish with a body length of optimal fishing length. The distribution of body length actually remains at the same level as that when fishing is terminated. However, the distribution does not return to its initial status, although it tends to approach the average body length. Therefore, individual genes likely disappear from a fish population exposed to a long-term gillnet overfishing. Consequently, these potential changes of genes will influence the hereditary traits of the population (Lianget al., 2012).

3 Further Studies on Life History Traits

Sustained environmental and fishing pressure will change the biological characteristics of most fish life histories (Law, 2000). Of these changes, that in body length is the most obvious. A number of investigators have hypothesized that acute environmental pressure prevents fish population growth. Other investigators have hypothesized that long-term fishing pressure changes the genetic traits of fish individuals (David, 2008; Heiseh, 2010). Both environmental and fishing pressure may cause the miniaturization of fish populations. Further research on the problem of fish life history is needed. Numerous experiments and numerical simulations of individual life history in fish population showed that fishing can be considered an important factor that affects the changes in the morphological traits of fish, and these changes induced the evolutionary on fish life history (Conover and Munch, 2002). It is necessary for scientists to clarify whether the effects of fishing on fish life history involve the changes of genetic mechanisms.

Although fish miniaturization is not caused entirely by fishing, many studies showed that fishing plays a major role in many harvested stocks (Jorgensenet al., 2009). Whatever the selective characteristics of the fishing gear, the harvester always prefers to capture large fish in target species. If the design of the fishing gear does not ensure that only large fish can be captured, the rational harvester will capture individuals of all sizes simultaneously and then discard the small fish of low commercial value. Currently, a widely adopted approach to fishery management advocates harvesting large fish and bypassing small fish (size-selective harvesting). According to size-selec-tive harvesting, the large individuals of particular species are preferentially taken, and this practice is common in both marine and terrestrial habitats (Phillip and Kaustu, 2008). However, the preferential removal of the large individuals of a species has been shown to have a negative effect on the demography, life history and ecology of that species (David, 2008).

Currently, fishery resources continue to collapse. Because protection is urgently needed, research on fish gear selectivity appears to be imperative and necessary. The need for such research will encourage theoretical studies in related areas and promote the sustainable use and protection of ocean fisheries resources. Current fishery management policies request that harvesters capture large-sized fish and release small-sized fish. However, Conover’s experiment indicates that this strategy will affect the characteristics of the evolution of the harvested fish population in a way that will prevent the population from recovering and returning to its original state (Conover and Munch, 2002). These results may explain why most endangered fish stocks did not recover as expected, even after fishery management approaches were adopted (Williams, 2004). A long time may be required for such recovery. The time required for the recovery of a fish population to its original state needs further study. Furthermore, the novel management concept of harvesting fish according to their morphological traits is proposed. The effect of this idea on the development of future fisheries and the fishing techniques that can support this management concept require further discussion and study.

Acknowledgement

We acknowledge the financial support from Special Fund for Agro-scientific Research in the Public Interest (No. 201203018).

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(Edited by Qiu Yantao)

(Received September 29, 2012; revised October 28, 2012; accepted March 13, 2013)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2014

*Corresponding author. Email: liang@ouc.edu.cn