A shift in hake (Merluccius merluccius) population of the NW Mediterranean induced by a combined effect of climate and fishery harvesting

Climate and fisheries are often interacting with each other and producing complex synergic effects on the population dynamics of marine species. The drastic change in the climatic conditions in the North Atlantic (NA) in the early eighties triggered changes in local weather and hydrographical conditions in the Mediterranean Sea. In the other hand, the long term fishery harvesting on the stock of European hake (Merluccius merluccius) off the Balearic Islands (BA, NW Mediterranean) changed the properties of the population and its structure in the early eighties, which could have triggered a different response to the environmental variability. Using time series analysis (on the environmental and hake CPUE time series) and a population simulation model, we examine the hypothesis that these processes are linked and, in turn, a combined effect of fishery harvesting and environmental changes in the NA induced an ecological shift in the hake population off BA. The results points out that the changes in the variance and modes of hake CPUE were consistent with changes in the hydroclimatic variability in the NW Mediterranean induced by the North Atlantic climate. Population simulations show that age-structured populations act like a filter of the environmental fluctuations and can intrinsically generate cycles (‘cohort-resonance effect’) consistent with the 12 yr mode observed in the CPUE before the eighties. Size selective fishing mortality could erode the age structure and change the population filter properties. The population would have changed from internally-generated fluctuations to an externally-forced fluctuations mode after the eighties, increasing its dependency on the recruitment variability. This fits with the observations which show the high dependency of the CPUE on the recruitment variability and the winter condition modes predominating after eighties. Our results emphasize the importance of the interaction between fisheries, environment and internal dynamics that can cause transitory shifts in the behaviour of a populatio

Regeneration potential of the Baltic Sea inferred from historical records

Overfishing of large predatory fish populations has resulted in lasting restructurings of entire marine food webs worldwide, with potential immense socio-economic consequences. Fortunately, some degraded ecosystems have started to show signs of regeneration. A key challenge for resource management is to anticipate the degree to which regeneration is possible, given the multiple threats ecosystems face. Here, we show that under current hydroclimatic conditions, complete regeneration of a heavily altered ecosystem –the Baltic Sea as case study– would not be possible. Instead, as the ecosystem regenerates it moves towards a new ecological baseline. This new baseline is characterized by lower and more variable biomass of the commercially important Atlantic cod, even under very low exploitation rates. Consequently, societal costs increase due to higher risk premium caused by increased uncertainty in biomass and reduced consumer surplus. Specifically, the combined economic losses amount to about 120 million € per year, which equals half of today’s maximum economic yield for the Baltic cod fishery. Our analyses suggest that shifts in ecological and economic baselines, in combination with increased biomass variability, lead to higher economic uncertainty and costs for exploited ecosystems, in particular under climate change.Kiel Cluster of Excellence 'Future Ocean

A combination of hydrodynamical and statistical modeling reveals non-stationary climate effects on fish larvae distributions

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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

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 pres- sure 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 IslandsPublicado

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

Population growth is affected by several factors such as climate, species interaction and harvesting pressure. However, additional complexity can arise if fishing increases the sensitivity to environmental variability. To predict the effects of fisheries and climate on marine populations, there is a need for improved understanding of how they affect key ecological processes such as population growth. In this study, we used a comparative approach investigating commercially fished species across different ecosystems: the Norwegian Sea−Barents Sea (Northeast Arctic cod), the North Sea (North Sea cod), the Atlantic Ocean (European hake), the Mediterranean Sea (European hake), and the Gulf of Alaska and Bering Sea (walleye pollock). Our objective was to compare the effects of commercial fisheries, age structure and environmental variability on population growth rate. We show that although all stocks experienced a decline in abundance, only 3 of them showed a concomitant decreasing trend in generation time (South Atlantic hake, North Atlantic hake and Northeast Arctic cod), suggesting a fishing-induced erosion in their age structure. Intra-specific analysis shows that changes in generation time triggered an increase in the relative contribution of recruitment to population growth. Furthermore, the contribution from recruitment to population growth changes due to large-scale climate indices or regional-scale environmental covariates, such as sea temperature. This study illustrates how and where the interaction between large-scale ecological patterns and regional/short-scale processes are important for designing management regulationsPublicado