Ecological consequences of body size decline in harvested fish species: positive feedback loops in trophic interactions amplify human impact

Abstract

Humans are changing marine ecosystems worldwide, both directly through fishing and indirectly through climate change. One of the little explored outcomes of human-induced change involves the decreasing body sizes of fishes. We use a marine ecosystem model to explore how a slow (less than 0.1% per year) decrease in the length of five harvested species could affect species interactions, biomasses and yields. We find that even small decreases in fish sizes are amplified by positive feedback loops in the ecosystem and can lead to major changes in natural mortality. For some species, a total of 4 per cent decrease in length-at-age over 50 years resulted in 50 per cent increase in predation mortality. However, the magnitude and direction in predation mortality changes differed among species and one shrinking species even experienced reduced predation pressure. Nevertheless, 50 years of gradual decrease in body size resulted in 1-35% decrease in biomasses and catches of all shrinking species. Therefore, fisheries management practices that ignore contemporary life-history changes are likely to overestimate long-term yields and can lead to overfishing.

Selective exploitation of spatially structured coastal fish populations by recreational anglers may lead to evolutionary downsizing of adults

“…These findings are in agreement with theoretical models and empirical data that found that size-selective recreational fishing leads to the evolution of high reproductive investment and small maximum body length , Saura et al 2010, Kendall & Quinn 2011. Such effects have been well recognized in size-selective commercial fisheries (Jørgensen et al 2007, Audzijonyte et al 2013b, and they can have negative implications for the utility that stocks provide to society (Eikeset et al 2013, Laugen et al 2014. We propose that recreational angling leads to lowered body size of adult stages similar to effects previously reported in size-selective commercial fisheries.…”

“…Based on our data, we propose that trait-selective and intensive recreational angling exploiting metapopulations of S. scriba constitute a potential mechanism that contributes to the decline of adult body length in heavily exploited coastal populations. The decline of the adult body size can have negative implications for the productivity and biomass of fish stocks (Jørgensen et al 2007, Audzijonyte et al 2013b). Both selective fishing and meta-population structure are, therefore, important to consider when managing coastal populations that are exploited by recreational anglers.…”

“…Until such research becomes available, our study proposes a new paradigm of the potential for widespread fisheriesinduced evolution of small-bodied coastal fishes at small scales. Due the potential negative effects of fisheries-induced evolution, which may negatively affect productivity, recovery and catch per unit effort of the stock (Sutter et al 2012, Audzijonyte et al 2013b, managers may adjust their management polices to prevent or decrease the selective impacts of harvesting for successful fish-stock management (Walsh et al 2006). …”

“…Fisheries-induced selection should select for fast life histories (Heino et al 2013) characterised by small size and age at maturation coupled with high reproductive investment, which collectively reduce the individual's adult body size (Law 2000, Jørgensen et al 2007). Any change in adult fish body size can have negative consequences for the productivity and utility of exploited systems (Jørgensen & Fiksen 2009, Audzijonyte et al 2013b, Jørgensen & Holt 2013. Consequently, knowledge about the selection differentials on life-history traits exerted by fisheries is important for sustainable fisheries management ).…”

Thresholds for impaired species recovery

“…This has been attributed to predation by grey seals (Halichoerus grypus) which have increased more than tenfold [64]. Mounting evidence suggests that the natural mortality of marine fishes can increase as abundance declines in overfished populations [44,[65][66][67]. And, all else being equal, increased mortality can readily generate an Allee effect in depleted populations [33].…”