Fish spawning strategies in the northwestern Mediterranean Sea

Abstract

The Mediterranean is globally considered an oligotrophic sea. However, there are some places or certain seasons in which mechanisms that enhance fertility may occur. These mechanisms, and related processes, are especially relevant in maintaining fish populations when they take place during the period of larval development. This contribution analyzes how environmental conditions occurring in the NW Mediterranean, at local and seasonal scales, determine the temporal and spatial patterns of fish reproductive activity in the region. The structure of the bathymetry, diversity of adult fish habitats and hydrodynamic mechanisms conditioning the primary production of the region (e.g., shelf-slope density front and associated current, continental water inflows, winter mixing, stratification of the water column) determine the location of spawning and the distribution patterns of fish eggs and larvae. A pronounced seasonal variability regarding both the number of species and the number of fish larvae in the plankton can be observed throughout the year. Most NW Mediterranean fish reproduce during the spring–summer stratification period, when the phytoplankton biomass values at the upper layers of the water column are lower than in winter. The development of the Deep Chlorophyll Maximum in this period and the high zooplankton biomass associated to it offers an important food source for the larvae. Additionally, during this period the inputs of continental waters are one of the fertilization mechanisms of surface waters and some species, as anchovy, takes advantage of this situation. Autumn–winter is the period with lower ichthyoplankton diversity, being dominated by sardine. Vertical mixing during winter is one of the mechanisms that enhance productivity. Overall, fish species show reproductive strategies and larval fish behavior that allow them to take advantage of the available resources throughout the seasonal cycle. These strategies, together with the high ecological efficiency of oligotrophic systems, contribute to the relatively high yield of Mediterranean fisheries. In a context of global change, understanding of the mechanisms relating environmental changes to the extent of spatial and temporal location of suitable spawning habitats of fish is a key first step to predicting and projecting such future changes, and thereby adapting to these changes.