T. Hakala Æ M. Viitasalo Æ H. Rita Æ E. Aro J. Flinkman Æ I. Vuorinen Temporal and spatial variation in the growth rates of Baltic herring (Clupea harengus membras L.) larvae during summer Received: 11 February 2002 / Accepted: 12 August 2002 / Published online: 26 September 2002 Ó Springer-Verlag 2002 Abstract It has been suggested that larval survival de- termines the year-class strength in most marine fish species. During their growth and development, the ability of the larvae to catch prey and avoid predation will increase. However, the factors affecting short-term changes in the growth of Baltic Sea herring have been little studied in the field. We collected Baltic herring (Clupea harengus membras L.) larvae from five different towing areas in the Archipelago Sea (SW Finland) during May and June 1989, right after the main spawning season. Twenty thousand two hundred and ten larvae were analysed and the area-specific growth rate (i.e. increase in standard length) was estimated by tracing the larval cohorts from the length–frequency data. This represents the first Baltic herring study with daily sampling during a long study period. The growth rate was related to environmental factors, such as temperature, number of zooplankters, and wind speed and direction. Large variation in larval growth rate occurred between areas: lowest and highest growth rates were 0.18 and 0.52 mmÆday –1 . Temperature was an important variable controlling larval-fish growth rate. An increase of one 1°C in average water temperature corresponded to an increase in growth rate of 0.043 mmÆday –1 . This may have been caused either by a direct temperature effect (changes in metabolic rate) or by the indirect effect of changes in food availability. We also found the densest herring populations in the areas with highest average water temperature. However, temperature and larval growth rate both increased towards the inner archipelago. Introduction In all fish species, growth rate defines the duration of the larval stage and therefore the length of time the larvae are susceptible to mortality factors, such as low tem- perature, low food availability, and predation (reviewed by Hempel 1963; Dragesund and Nakken 1971; Buckley et al. 1987; Busch et al. 1996). The ability of larvae to catch prey and avoid predation will increase during their growth and development (e.g. Blaxter 1986). Hjort (1914) presented the ‘‘critical period’’ concept, which holds that the year-class strength of fish is largely determined by survival during the early larval stage. According to Blaxter and Hempel (1963), the transition period from yolk-sac stage to external feeding is an es- pecially critical period. The failure to initiate external feeding can lead to a ‘‘point of no return’’ when the starved larva becomes too weak to feed even if enough food is available. Finally, Cushing (1975) concluded that the success or failure of the year-class is a result of the temporal ‘‘match or mismatch’’ between the abundance of the larvae and that of their prey. Year-class strength of herring (Clupea harengus harengus) has been observed to fluctuate strongly from Marine Biology (2003) 142: 25–33 DOI 10.1007/s00227-002-0933-3 Communicated by L. Hagerman, Helsingør T. Hakala (&) Department of Ecology and Systematics, Division of Hydrobiology, P.O. Box 17, University of Helsinki, 00014 Helsinki, Finland E-mail: tomi.hakala@erweko.fi Tel.: +358-40-5538422 M. Viitasalo Æ J. Flinkman Finnish Institute of Marine Research, P.O. Box 33, 00931 Helsinki, Finland H. Rita Department of Ecology and Systematics, Division of Population Biology, P.O. Box 17, University of Helsinki, 00014 Helsinki, Finland E. Aro Finnish Game and Fisheries Institute, Fisheries Division, P.O. Box 6, 00721 Helsinki, Finland I. Vuorinen Archipelago Research Institute, University of Turku, 20014 Turku, Finland T. Hakala Keijuniityntie 5 C 14, 02130 Espoo, Finland