The biology of blue whiting (Micromesistius poutassou) in the NW Mediterranean: a species under siege by overfishing and climatic constraints

Abstract

[eng] Blue whiting, Micromesistius poutassou (Risso, 1827), is a mesopelagic gadoid widely distributed in the North Atlantic and the Mediterranean Sea, inhabiting waters over the shelf edge and continental slope. The species plays an important role in the marine food web feeding on pelagic crustaceans and mesopelagic fish and being consumed by a wide range of predators from pelagic and benthic habitats. It is a species of commercial interest, exploited in the northern colder areas of the Mediterranean. Despite its ecological and commercial significance, information on its biology and behaviour is mostly restricted to the Atlantic waters, and the information for this species in the Mediterranean is very scarce. In the last decade, blue whiting population has displayed a decreasing trend in the NW Mediterranean, and now it remains at very low levels. In this context, this Ph.D. thesis aims to unveil the main drivers (physical, biological and anthropogenic) of the population dynamics of blue whiting in the NW Mediterranean Sea.

The blue whiting population parameters have been updated and the growth of the species and its relationship among reproduction and energy reserves have been described. Blue whiting reaches the first maturity at around 18 cm TL for both sexes that is achieved the spawning season following its birth. Females and males show different growth curves, with females reaching higher sizes than males. Spatial segregation by age and size has been observed. While smaller individuals (age 0 and 1) remain in the shelf break (130-250 m depth), the large ones (age >2) are more abundant in the upper slope (250-550 m). The largest individuals (>26 cm TL, age >3) display movements along the year and approach the coast in winter for reproduction, disappearing in summer and autumn. The study of the feeding ecology highlight the relevance of the Myctophidae fish as the main prey for blue whiting. This energetic prey is mainly consumed in spring and summer, which might allow the species to recover energy reserves after the reproductive period and supply the energetic demands of the fast growth season. The dominance of fish in the blue whiting diet evidences that the trophic position of the species in the Mediterranean is higher than in the Atlantic, where it mainly prey on Euphausiacea and zooplanktonic crustaceans.

The early life stages of blue whiting have been investigated to unveil the drivers of the recruitment strength of the species. The study of the trophic ecology of blue whiting larvae showed that the species modulates the number and size of ingested prey to meet the energetic requirements throughout its early ontogeny. The smallest larvae fed on small prey, Tintinnina and nauplii, shifting to larger more energetic prey, Calanoida copepods, as development progressed. From the flexion stage onwards, larvae prey exclusively on Calanoida, and no major switch in prey size was observed due to the scarcity of larger Calanoida in the field. This limitation forces the larger larvae to increase the number of ingested prey to meet their energetic demands. The feeding pattern of larvae was quite consistent in the two studied years, and similar to that reported for the species in the Atlantic, suggesting that they feed efficiently in the Mediterranean trophic environment. This invited further research to unveil the environmental processes acting on larvae or pre-recruiting juveniles to determine the variability in the recruitment strength. The comparison between two years of contrasted environmental winter conditions reveals that severe winters enhance the recruitment strength. Winter 2017 was mild, with a short period of cold temperatures and limited vertical mixing of the water column. In winter 2018, temperatures reached lower values and extended for a longer period with intense vertical mixing and cascading events. These different conditions were mirrored in the subsequent phytoplanktonic bloom, which in 2018 occurred later and was more intense, extending over a longer period and occupying a wider area, than in 2017. Whether it was the longer period of low temperatures, that results in a extended breeding season, the higher surface productivity, that would enhance the trophic resources for juveniles, or a combination of the two processes, severe winters conditions translate into improved recruitment of blue whiting.

The fishery dynamics of the species has been characterized to determine its role in the current state of the blue whiting population of the NW Mediterranean. The results show that it is a highly discarded

species, with maximum highest discard rate during the recruitment period, mainly in April-May (more than 95% of the total catch in number). A decline in landings has been observed in the last decade, probably related to the absence of severe winter conditions in the last years which enhance the recruitment of the species. However, the effect of the exploitation on the landings decrease cannot be disregarded considering that the bulk of the catch is made up of immature (age 0) individuals. Overall, the results suggest that the cumulative effects of fishing and climatic trends have led to the current low abundance levels of blue whiting compromising its success in the Mediterranean.

[cat] La maire, Micromesistius poutassou (Risso, 1827), és un gàdid mesopelàgic que es troba àmpliament distribuït a l’Atlàntic Nord i al mar Mediterrani. L’espècie te un rol ecològic important i és explotada comercialment. Tot i la seva rellevància, la informació sobre l’espècie està centrada principalment en les poblacions de l’Atlàntic, i la informació per al Mediterrani és molt escassa. Durant l’última dècada, la població de maire al Mediterrani nord-occidental ha anat disminuint i actualment es manté a nivells molt baixos d’abundància. En aquest context, aquesta tesis pretén dilucidar els principals impulsors (físics, biològics i antropogènics) de la dinàmica poblacional de la maire al Mediterrani nord-occidental.

En la tesi s’actualitzen els paràmetres poblacionals de la maire i es descriu el creixement de l’espècie i la relació amb la reproducció i les reserves energètiques. També s’ha estudiat l’ecologia tròfica de l’espècie, que ha destacat la importància dels peixos mictòfids com a presa principal de la maire al Mediterrani. S’han investigat els primers estadis de vida de la maire per esbrinar els factors que determinen la força de reclutament de l’espècie. L’estudi de l’ecologia tròfica ha mostrat que els patrons d’alimentació de les larves de maire han estat molt regulars els dos anys estudiats, i similars als trobats a l’Atlàntic, la qual cosa suggereix que les larves s’alimenten eficientment en l’ambient tròfic Mediterrani. Aquest resultat va portar a centrar el focus en els processos ambientals que, actuant sobre les larves o juvenils pre-reclutes, acaben determinant la variabilitat del reclutament. L’estudi comparat de dos anys amb condicions ambientals molt contrastades ha revelat que les condicions severes d’hivern afavoreixen el reclutament de l’espècie al Mediterrani nord-occidental. Finalment, s’estudia la dinàmica de la pesqueria de l’espècie els últims 20 anys per a entendre el seu paper en l’estat actual de la población. Els resultats mostren que el descens de les captures observat durant la darrera dècada probablement estigui relacionat amb l’absència d’hiverns severs en els últims anys, tanmateix però, l’efecte de l’explotació pesquera en la disminució de la població no pot ser menystingut si es considera que la major part de la captura està formada per individus immadurs (edat 0). En conjunt, els resultats d’aquesta tesi suggereixen que els efectes acumulats de la pesca i les tendències climàtiques han portat a l’actual baix nivell poblacional de l’espècie, comprometent el seu futur en el Mediterrani nord-occidental.