Population effects and changes in life history traits in relation to phase transitions induced by long-term fishery harvesting: European hake (Merluccius merluccius) off the Balearic Islands

Abstract

In 1980 and 1995, the European hake ( Merluccius merluccius ) population off the Balearic Islands (northwestern Mediterranean) underwent changes at both the individual level and the population level. There was a sharp decrease in abundance that coincided with a change in the seasonal catch-per-unit-effort pattern in 1980. A new population scenario emerged after 1980 characterized by an increase in the intrinsic growth rate and a decrease in carrying capacity; however, catchability remained the same. An age-structure truncation could have caused these changes, making the population more dependent on year-to-year recruitment. A change in size structure also occurred in 1995, which was evidenced by a sudden decrease in the mean length-at-age (L) and abundance of recruits and a change in the density-dependent effect on recruits. As the Mediterranean trawl fishery mainly harvests recruits and juveniles and fishery harvesting induces adaptive changes in life history traits, the sharp decrease in L of recruits could be explained as a growth reduction due to the selective pressure to stay under mesh size for longer and thus maximize survival until reproduction. These individual and population transitions explain the changes in the response to the environmental forcing observed in the European hake off the Balearic Islands.

Synergistic effects of fishing-induced demographic changes and climate variation on fish population dynamics

“…Effort estimates displayed a constant upward trend from 1940 to the early 1990s, after which they began a downward trend. Fluctuations in CPUE are independent of the trawl effort (Oliver 1993) and consistently represent the dynamics in the EH population , Hidalgo et al 2009a.…”

“…1). Previous investigations reported 2 major changes in the inter-annual variability of population abundance in EH, in 1980 and 1995, and attributed them to the long-term exploitation of this species (Hidalgo et al 2009a). Mediterranean fisheries are known for their long-term harvesting of juveniles, with EH being the most representative species showing this exploitation pattern (Caddy 1993).…”

Balearic Islands vs Algeria: two nearby western Mediterranean elasmobranch assemblages with different oceanographic scenarios and fishing histories

“…This last species is considered as extremely vulnerable to fishing impacts even within the general context of high elasmobranch vulnerability (Coelho and Erzini, 2008), and was much more abundant in the slope assemblages off Algeria than in those off the Balearic Islands. Although the bottom trawl survey series in the Balearic Islands is relatively short (10 years, including late springsummer surveys), E. spinax shows a clear decline in abundance in the red shrimp fishery of that area, indicating that the displacement of the fishing effort to these bottoms during the last decades (Hidalgo et al, 2009) could have started to show its effects on the most vulnerable species. Caution should be taken with these results, because G. melastomus shows no significant trend for the same period.…”

“…The red shrimp fishery off the Balearic Islands is located on the middle slope below 500 m depth and has undergone an increase in the trawl fishing effort during the last few decades (Hidalgo et al, 2009, Palmer et al, 2009. In order to determine whether this increase has started to affect the elasmobranch species inhabiting these bottoms, linear regression was fitted to the relationship between the mean abundance, standardized to one square kilometre, of the most important elasmobranch species and year of the survey.…”

Twenty-five-year longevity of European hake (Merluccius merluccius) from novel use of bomb radiocarbon dating in the Mediterranean Sea

“…This indicates the full life history of European hake could be a combination of rapid early growth followed by a prolonged lifespan. In this regard, it is worth noting that the longevity reported herein refers to fish born in the 1960s and the increase of fishing effort over the past 50 years could have affected biological traits (Hidalgo et al 2009). Moreover, throughout the Mediterranean, this species inhabits heterogeneous habitats characterised by a wide range of abiotic and biotic factors that might induce adaptive changes in growth.…”