Vertical distribution and migration of fish larvae in the NW Iberian upwelling system during the winter mixing period: implications for cross‐shelf distribution

Abstract

The vertical distribution and vertical migrations of fish larvae and implications for their cross-shelf distribution were investigated in the northern limit of the NE Atlantic upwelling region during the late winter mixing period of 2012. The average positive values of the upwelling index for February and March of this year were far from normal, although the average hydrographic conditions during the period of study were of downwelling and the water column was completely mixed. Fish larvae, most in the preflexion stage, were concentrated in the upper layers of the water column and their distribution was depth stratified, both day and night. However, the larval fish community was not structured in the vertical plane and fish larvae did not show significant diel vertical migration (DVM), although five species showed ontogenetic vertical migration. In regions of coastal upwelling and in the absence of DVM, the location of fish larvae in the water column is crucial for their cross-shelf distribution. Thus, the cross-shelf distribution of the six most abundant species collected in this study can be explained by the surface onshore flow associated with coastal downwelling, retaining larvae of the coastal spawning species with a relatively shallow distribution in the shelf region and transporting larvae of slope spawning species onto the shelf. The wide vertical distribution shown by larvae of the offshore spawning species could be an adaptation of these species to ensure that some larvae reach the inshore nursery areas.

Composition and structure of the larval fish community in the NW Iberian upwelling system during the winter mixing period

“…data). For its part, the vertical distribution of larval sizes of M. merluccius, M. poutassou, M. variegatus and T. minutus found during this study suggests ontogenetic ascent in the water column (Rodriguez et al 2015). This upward movement could place larger larvae within the Ekman layer associated with coastal downwelling and therefore favour their retention (neritic spawning species) or their coastward transport (oceanic slope-spawning species).…”

“…Despite the variability in wind speed and direction and the resulting variability in the hydrographic conditions, Rodriguez et al (2015) concluded that the average hydrographic conditions during the period of study were of downwelling. The higher concentration of fish larvae found in the frontal region was consistent with downwelling conditions during this time.…”

“…However, Rodriguez et al (2015) found that mesozooplankton and fish larvae were relatively segregated in the vertical plane. Thus, the relationship between these plankton components could have arisen from the horizontal coincidence between them.…”

“…S. pilchardus, S. scombrus and Trisopterus minutus) near shore and by spreading larvae of slope spawning species (e.g. Maurolicus muelleri, Merluccius merluccius and Micromesistius poutassou) over the shelf (Rodriguez et al 2015), would account for the observed patterns of crossshelf distribution shown by fish larvae. The concentration of larger larvae of coastal-shelf spawning species in the 100−200 m depth stratum would support their retention over the shelf.…”

Study of sardine (Sardina pilchardus) regime shifts in the Iberian Atlantic shelf waters

“…deCastro, Gómez‐Gesteira, Álvarez, and Crespo () detected intensification in the IPC in January 1989, 1990, 1996, and 1998. Downwelling conditions help to spread the IPC over the shelf that could contribute to larval retention (Rodriguez et al., ; Santos et al., ), but if this persists for a long time, the decline in nutrients input from upwelled water could be a limiting factor for larvae growth and survival (Cury & Roy, ).…”