Why fishing magnifies fluctuations in fish abundance

Abstract

It is now clear that fished populations can fluctuate more than unharvested stocks. However, it is not clear why. Here we distinguish among three major competing mechanisms for this phenomenon, by using the 50-year California Cooperative Oceanic Fisheries Investigations (CalCOFI) larval fish record. First, variable fishing pressure directly increases variability in exploited populations. Second, commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly. Third, age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates. We find no evidence for the first hypothesis, limited evidence for the second and strong evidence for the third. Therefore, in California Current fisheries, increased temporal variability in the population does not arise from variable exploitation, nor does it reflect direct environmental tracking. More fundamentally, it arises from increased instability in dynamics. This finding has implications for resource management as an empirical example of how selective harvesting can alter the basic dynamics of exploited populations, and lead to unstable booms and busts that can precede systematic declines in stock levels.

Commercial coral-reef fisheries across Micronesia: A need for improving management

“…While Pohnpei has the greatest potential for reef fisheries among the jurisdictions examined, the disproportionally high harvest yields, environmentally driven success, and previously published doctrine collectively suggest current harvesting levels are unsustainable. Coupling our results with the existing doctrine, predicted reductions in trophic levels, catch-per-unit-effort, and ecological function are posited without a shift in harvesting patterns and policies (Jennings and Polunin 1996;Dulvy et al 2004;Graham et al 2005;Anderson et al 2008a;McClanahan 2010). This situation in Pohnpei appears to be driven, in part, by an artificially low cost for reef fish.…”

“…Yet, even on Yap where human population is lowest and reef size is moderate, many desirable species were harvested below their mean reproductive sizes, including the iconic green bumphead parrotfish and humphead wrasse, as well as several other common herbivores. The collective trends have known affiliations with high fishing effort, decreased catch-per-unit-effort, less sustainable fishery yields, diminished stocks, and reduced ecological function on coral reefs (Jennings and Polunin 1996;Pauly et al 1998;Jennings and Greenstreet 1999;Graham et al 2005;Cinner and McClanahan 2006;Anderson et al 2008a;Lokrantz et al 2008;McClanahan 2010). Clearly, there is a high demand for reef fish throughout Micronesia, and as demand for any exhaustible resources grows, exploitation intensity increases (Hotelling 1991).…”

“…We initially examine the degree to which favorable environmental regimes, proxies of fishing intensity, and marketed fish value explained the observed catch dynamics. We hypothesize that a higher reliance upon favorable environments for good catches, and higher variability in daily landings coincide with increased fish demand (Anderson et al 2008a;McClanahan et al 2008). We further posit that reduced trophic levels and decreased mean size-at-capture for targeted reef fish will be most pronounced where high proxies to fishing pressure exist (Jennings and Polunin 1996;Jennings and Greenstreet 1999;Graham et al 2005).…”

“…Disentangling the relationships between shortterm seasonal changes and long-term trends continues to be a central focus for fisheries research (May et al 1979;Hsieh et al 2006;Anderson et al 2008a). The current doctrine supports that as fishing intensity increases, shifts in population and community structure become evident (Jennings and Greenstreet 1999;Graham et al 2005), and stock variability increases due to a tightening of feedback loops between environmental regimes and fish stocks (Anderson et al 2008a). In summary, desirable fish landings become more reliant upon favorable environmental and/or temporal regimes and eventually decline as this reliance builds.…”

“…While environmental patterns are known to influence fisheries dynamics over short time-scales (i.e., moon phase within a month or productivity within a year; McClanahan 1988; Kaunda-Arara et al 2009), long-term dynamics are more dependent upon fishing effort and individual species' life histories (Pauly et al 1998;Tsehaye et al 2007). Disentangling the relationships between shortterm seasonal changes and long-term trends continues to be a central focus for fisheries research (May et al 1979;Hsieh et al 2006;Anderson et al 2008a). The current doctrine supports that as fishing intensity increases, shifts in population and community structure become evident (Jennings and Greenstreet 1999;Graham et al 2005), and stock variability increases due to a tightening of feedback loops between environmental regimes and fish stocks (Anderson et al 2008a).…”