Evolutionary response to size-selective mortality in an exploited fish population

Abstract

Many collapsed fish populations have failed to recover after a decade or more with little fishing. This may reflect evolutionary change in response to the highly selective mortality imposed by fisheries. Recent experimental work has demonstrated a rapid genetic change in growth rate in response to size-selective harvesting of laboratory fish populations. Here, we use a 30-year time-series of back-calculated lengths-at-age to test for a genetic response to size-selective mortality in the wild in a heavily exploited population of Atlantic cod (Gadus morhua). Controlling for the effects of density- and temperature-dependent growth, the change in mean length of 4-year-old cod between offspring and their parental cohorts was positively correlated with the estimated selection differential experienced by the parental cohorts between this age and spawning. This result supports the hypothesis that there have been genetic changes in growth in this population in response to size-selective fishing. Such changes may account for the continued small size-at-age in this population despite good conditions for growth and little fishing for over a decade. This study highlights the need for management regimes that take into account the evolutionary consequences of fishing.

Data from selective harvests underestimate temporal trends in quantitative traits

“…In our study, this situation was exacerbated by regulations making it illegal to harvest small males. Similar preferences for harvesting larger individuals are well known in fisheries [6,13]. For example, size-selectivity of fisheries for cod (Gadus morhua) in the Gulf of St Lawrence resulted in fast-growing fish being more likely to be caught than slow-growing fish.…”

“…For example, size-selectivity of fisheries for cod (Gadus morhua) in the Gulf of St Lawrence resulted in fast-growing fish being more likely to be caught than slow-growing fish. Consequently, data from declining commercial fisheries might underestimate the decline in age-specific mass for the population [6]. Base circumference began to decline about 5 years later than horn length, and the difference in temporal decline between all rams and harvested rams was not as evident as for horn length.…”

“…As humans often prefer sex-age classes or morphological traits associated with high natural survival, harvest mortality differs from natural mortality [5]. Thus, harvests may lead to evolutionary responses in life histories and morphology [6], outpacing other selective agents [7,8]. There have been calls to consider potentially undesirable artificial selection in management and conservation [9,10].…”

Holistic Fisheries Management: Combining Macroecology, Ecology, and Evolutionary Biology

“…These effects are exemplified by changes in growth rates in response to size-selective fishing (Swain et al, 2007). However, the direct genetic impacts of harvesting are not alone.…”

Change in individual growth rate and its link to gill-net fishing in two sympatric whitefish species

“…Fishing is typically size-selective (Myers and Hoenig 1997;Fukuwaka and Morita 2008), and since significant heritabilities have been reported for traits that could be size-related in many fish [up to h 2 = 0.5, see (Theriault et al 2007;Carlson and Seamons 2008)], size-selective fishing is expected to induce rapid evolutionary changes (Palumbi 2001;Smith and Bernatchez 2008;Darimont et al 2009). Traits such as age or size at maturation (Heino et al 2002;Grift et al 2003;Sharpe and Hendry 2009), average reproductive effort (Yoneda and Wright 2004;Thomas et al 2009), or individual growth rates (Handford et al 1977;Ricker 1981;Swain et al 2007; Thomas and Eckmann 2007;Nusslé et al 2009) are likely to evolve in response to size-selective fishing. Such fishing has therefore been termed a 'large-scale experiment in life-history evolution' (Rijnsdorp 1993;Law 2000;Stokes and Law 2000), and studies on fishery-induced evolution have increased in numbers during the last decade (see Jørgensen et al (2007) for a review of phenotypic change attributed to fishery-induced selection).…”