Influence of the Seasonal Thermocline on the Vertical Distribution of Larval Fish Assemblages Associated with Atlantic Bluefin Tuna Spawning Grounds

Abstract

Temperature is often an important variable influencing the vertical position of fish larvae in the water column. The same species may show different vertical distributions in areas with a strong near-surface seasonal thermocline compared to isothermal near-surface regions. In areas with a strong surface thermocline, tuna larvae show a significant preference for the near-surface warmer layers. Little is known regarding larval tuna vertical distribution in isothermal waters and on the vertical distribution of the associated larval fish assemblages. We conducted vertical stratified sampling using the same methodology and fishing device (MOCNESS) in the two major spawning areas of Atlantic bluefin tuna (BFT): western Mediterranean Sea (MED), characterized by a surface thermocline, and the Gulf of Mexico (GOM) which lacks thermal stratification. Tuna larvae occupied the upper 30 m in both areas, but the average larval depth distribution was consistently deeper in the GOM. In the MED, vertical distribution of larval fish assemblages was explained by temperature, and species such as BFT, Thunnus alalunga, and Ceratoscopelus maderensis, among others, coexist above the thermocline and are separated from species such as Cyclothone braueri and Hygophum spp. (found below the thermocline). In the GOM, the environmental correlates of the vertical distribution of the larvae were salinity and fluorescence. Mesopelagic taxa such as Ceratoscopelus spp. and Cyclothone spp., among others, had a shallower average distribution than Lampanyctus spp., Hygophum spp., and Myctophum spp.

Differential larval growth strategies and trophodynamics of larval Atlantic bluefin tuna (Thunnus thynnus) from two discrete spawning grounds

“…Comparable to Alemany et al (2010), when larval ABT overlapped with other scombrids, it was most often with the congener (T. alalunga) and with Auxis spp. The trends observed within the same 2014 spawning season confirm previous larval assemblage comparisons from different years observed for scombrids between the GoM and MED Alvarez et al, 2021) and within the MED alone (Uriarte et al, 2019).…”

Fish sounds of photic and mesophotic coral reefs: variation with depth and type of island

“…This study also indicates that acoustic fish communities, from the surface to 120 m, are divided into two parts with a buffer zone around 60 m. This buffer zone seems to coincide with a zone showing a higher fish species richness on the drop-off (Bertucci et al 2017). In addition, another explanation of this division is likely related to temperature and more precisely to the presence of one to several thermoclines, known to influence fish assemblage composition (Alvarez et al 2021). Finally, depth-dependent differences in acoustic fish communities are also related to changes in the benthic cover composition (Di .…”

“…Diagnostic plots were used to verify assumptions. Because temperature data was not available for all the islands, the effect of temperature could not be tested, but it was discussed, as it is known to influence fish assemblage composition (Alvarez et al 2021).…”

Spatial Dynamics of Demersal Fish Assemblages off the Korean Coast in the East Sea

“…Although there were seasonal differences in the position of the thermocline, this study cannot detect any influences of seasonal thermocline depth on demersal fish assemblage. Nonetheless, since seasonal changes in thermocline can affect the vertical distribution of fish assemblage [ 65 , 66 ], it is important for further studies to dig deeper and confirm how much of an impact these seasonal shifts really have on fish assemblages. Understanding the composition of faunal assemblages in coastal habitats lays important baselines for future research and management strategies, particularly in regions like the eastern Korean waters, where research is currently limited.…”