Fishing impact amplifies the sensitivity of exploited marine populations to climate variability

May 16, Palma de Mallorca. Scientists from the Oceanographic Centre of the Balearic Islands of the Spanish Institute of Oceanography (IEO) in collaboration with the University of Oslo and the Institute of Marine Sciences, Leibniz (IFM-GEOMAR), have shown that the effects of fishing and climate on fish populations are not independent and, therefore, cannot be studied separately as it has been done for long time.

The study, published as feature article in the first volume of April in the journal Marine Ecology Progress Series, examines the time series of abundance index (catches per fishing boat unit effort) of European hake (Merluccius merluccius) in the Balearic Islands since 1940. This data series, one of the longest in the Mediterranean, contains periodic information on catches and fishing effort, collected in several research projects carried out by the Oceanographic Centre of the Balearic Islands in collaboration with the fishing industry. This time series, which displays a clear oscillatory pattern, was analyzed using hydroclimatic variables along the Balearic Sea area, being the results reproduced by simulations of the population dynamics.

The study shows that, in absence of overexploitation, the fluctuations in the abundance of hake in the Balearic Islands were independent of environmental changes. As the fishing exploitation has been eroding the demographic structure by eliminating individuals of larger size and age, the population has been progressively more dependent on younger individuals, a process known as demographic truncation. In this situation, population depended more on the annual incoming of juveniles from the spawning, a process highly sensitive upon the climate variations known as recruitment. The consequence was the loss of the natural capacity for the population to rely on its own dynamics, increasing, therefore, its sensitivity to environmental changes.

These results have enabled us to understand why extreme changes in hydroclimatic conditions in the western Mediterranean, occurred in the early 80's, caused a sharp decline in hake stocks of hake in the Balearic Islands. The intense overexploitation of the species led to a population mainly supported by juveniles and young spawners, a demographic structure highly sensitive to climate variations.

The conclusions of this study are important for the future management of marine resources. While populations with a healthy demographic structure are easy to manage, the management of populations with an eroded demography is more complicated due to its high dependence upon the unpredictable environmental variability.