Abstract
Growth and survival of Maurolicus muelleri larvae in Herdlefjorden, Norway, were investigated by daily otolith increment analysis. While high egg densities were generally observed throughout the spawning season, three cohorts each with a narrow window of hatching dates were identified. The first of these cohorts was characterized by low growth and poor morphometric condition and disappeared from the fjord during autumn. High-resolution drift modeling indicated that Herdlefjorden had a net export of larvae and negligible import in the period cohort 1 disappeared. Yet, the advective loss rate of larvae was not considered high enough to explain the near complete disappearance of the first cohort. An otolith-based growth chronology indicated that growth conditions in Herdlefjorden improved noticeably around mid-September and remained favorable the following month. The analysis of daily otolith increments could thus be used to document within-season variability in larval growth and survival. The low and variable survival due to short-term fluctuations in environmental conditions indicate that multiple batch spawning is an adequately evolved life-history strategy for marine planktivorous fish such as M. muelleri.
Highlights
A hundred years ago, Johan Hjort initiated a paradigm shift in fisheries ecology by documenting year class variation in boreal fish species by means of scale increment analysis (Hjort 1914)
Half of all larvae caught at the last sampling had hatched within a window of 10 days, and 75% had hatched within 20 days
Larvae hatched during the five first months of the spawning season do not seem to have contributed to the 1995 year class of M. muelleri in the fjord (Fig. 2)
Summary
A hundred years ago, Johan Hjort initiated a paradigm shift in fisheries ecology by documenting year class variation in boreal fish species by means of scale increment analysis (Hjort 1914). Analyses of annual biochronological data in scales and otoliths have been routinely used in age-based fisheries assessments. The pattern of annual structures in scales and otoliths was convincing in documenting annual variations in recruitment, no firm conclusion could be drawn as to what were the underlying mechanisms behind this variation. The discovery of daily otolith increments by Pannella (1971) opened up the possibility of documenting within-season variation in growth and survival of larval fish. Otolith microstructure analysis provided a new method for documenting how and why recruitment varied between years. Several authors have subsequently used this methodology to infer regional, seasonal, and size-related differences in growth and survival (e.g., Methot 1983; Meekan and Fortier 1996; Baumann et al 2003; Sponaugle 2010)
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