Baumann, Hannes, Voss, Rüdiger, Hinrichsen, Hans-Harald, Mohrholz, V., Schmidt, Jörn and Temming, A. (2008) Investigating the selective survival of summer- over spring-born sprat, Sprattus sprattus, in the Baltic Sea. Fisheries Research, 91 (1). pp. 1-14. DOI 10.1016/j.fishres.2007.11.004.

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Abstract

To better understand recruitment variability in small pelagic fish like sprat, it is important to know when during the extended spawning season the successful recruits are predominantly produced and which environmental factors determine potential survival windows. Here, we inferred the temporal origin of 2-year classes (2002–2003) of western and central Baltic sprat by means of otolith microstructure analysis, and found that in both years recruits mainly originated from the summer months June and July. In both years, this period coincided with temperature conditions in the surface layer of >12 °C and peak seasonal abundance of the largest copepod stages of Acartia spp., the major prey item of sprat larvae. The peaks in seasonal sprat egg abundance, however, occurred in April 2002 and March 2003 and therefore about 1–2 months earlier than the long-term mean spawning peak of sprat in this area (end of May/beginning of June). We hypothesize that increased temperatures in the bottom layer of the Baltic, where the pre-spawning sprat stock concentrates during winter months, potentially caused this shift in sprat spawning patterns, although early spring temperatures and feeding conditions in upper water layers were still unfavourable for larval survival. Sprat recruitment, however, was comparatively strong in both 2002 and 2003, suggesting that summer born individuals had high enough survival rates to compensate for the spawning shift, possibly due to high summer temperatures, limited dispersion, and low predation mortalities by Baltic cod as the major predator of sprat. Recruits were on average younger in 2003 than 2002, yet length distributions in October were almost identical, likely because a period of substantially higher temperatures in July/August 2003 promoted faster initial (larval) growth of survivors. Given the strength of the 2003 year class, in spite of lower overall prey concentrations in 2003 than 2002 in the study area, our findings appear to emphasise the paramount importance of summer temperatures as the recruitment determinant in Baltic sprat

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