Context‐dependent interplays between truncated demographies and climate variation shape the population growth rate of a harvested species

Abstract

Fisheries ecologists traditionally aimed at disentangling climate and fishing effects from the population dynamics of exploited marine fish stocks. However, recent studies have shown that internal characteristics and external forcing (climate and exploitation) have interactive rather than additive effects. Thought most of these studies explored how demographic truncation induced by exploitation affected the response of recruitment to climate, identifying a general pattern revealed to be difficult as interactions are often case-specific. Here we compared five exploited stocks of European hake Merluccius merluccius from the Atlantic Ocean and Mediterranean Sea to investigate how the interaction between internal characteristics and external forces affect the variability of the population growth rate and their consequences on recruitment. Our results show that demographic truncation induces a novel population scenario in which the growth rate is maximized when the reproductive stock is younger and less diverse. This scenario is shaped by the climate variability and the fishing pattern. The population growth rate becomes more dependent on the maturation schedule and less on the survival rates. The consequences for the recruitment dynamics are twofold; the effect of density-dependent regulatory processes decreases while the effect of the density-independent drivers increases. Our study shows that the interaction between internal characteristics and external forces changes across geographic locations according to 1) the importance of demographic truncation, 2) the influence of the climate on the regional hydrography and 3) the spatiotemporal heterogeneity of the physical environment to which fish life history is adapted.

Population growth across heterogeneous environments: effects of harvesting and age structure

“…Our findings emphasise how the elasticity to recruitment can be affected by a long-term change in age structure of the population (perhaps due to fishing; Hidalgo et al 2011, 2012, but see Ottersen et al 2013, but can also be directly affected by the ongoing fishing intensity. The elasticity of the population growth rate to recruitment cannot thus be solely explained by the age structure or the fishing intensity.…”

“…The temperature variables used have been reported to explain the dynamics for NA, SA and BI hake (Hidalgo et al 2012), NEA (Hjermann et al 2004) and NS cod (Hjermann et al 2013), and GOA (Ciannelli et al 2004) and EBS pollock (Mueter et al 2011). Sea surface temperature (SST) for hake were obtained from the Climate Diagnostics Center (NCEP/NCAR) reanalysis fields (Kalnay et al 1996) on a 1 × 1 degree grid and averaged over the area of each studied stock.…”

“…A positive effect of SST on NA and SA hake elasticity was shown in in creased larval survival, which thus favoured recruit ment success (Goikoetxea 2011, Hidalgo et al 2012). Similar to Hidalgo et al (2012), we found evidence for environ-mental effects on the elasticity to recruitment for SA hake (upwelling during summer and winter temperature) (Domínguez-Petit et al 2008) and (winter SST) for BI hake The 2 studied pollock stocks displayed opposite effects of SST on the elasticity to recruitment ( Table 2) that can be explained by the much lower temperature in the Bering Sea (SST May to July) compared with the Gulf of Alaska (SST sum1 June to August). In those areas, the climate effects could also trigger changes in trophic interactions, such as the recruitment of competitive species, or changes in spatial distribution (Bailey 2000, Ciannelli et al 2005.…”

Age-structure, harvesting and climate effects on population growth of Arcto-boreal fish stocks

“…In our study, the pre-eminent factor is the negative association between the generation time μ and the transient elasticity to recruitment, confirming a previous study conducted on other data for gadoids only ). This emphasises how the elasticity to recruitment can be affected by a long-term deterioration of the age structure of the population (Rouyer et al 2011, Hidalgo et al 2012. In other words, the reduction in generation time μ may trigger an increase in the relative contribution of recruitment to population growth.…”

“…Thus, year-to-year recruitment variability has a greater influence on short-lived species and a lesser impact on long-lived species where the mature population typically consists of a larger number of cohorts (Stearns 1992). However, changes in life history para meters (survival and age of maturation) are also ex pected to change the population's sensitivity to year-to-year recruitment variability (Rogers et al 2011, Hidalgo et al 2012) as expected in harvested populations. In recent de cades, the most conspicuous change is the large extra mortality exerted on exploited fish populations.…”