Chapter 9: Collecting, Processing, and Identification of Fish Eggs and Larvae and Zooplankton
William E. Kelso, Michael D. Kaller, and D. Allen Rutherford
This chapter is an introduction to methods for collecting, processing, and identifying planktonic animals that typically inhabit pelagic, benthic, and macrophytic habitats. It also summarizes the diverse studies on ichthyoplankton and zooplankton in both marine and freshwater environments. We review the gears that are used to collect these organisms, their relative effectiveness in different sampling situations, and the potential effects of aquatic physicochemistry (e.g., turbidity, dissolved oxygen concentrations, and habitat complexity) and organismal behavior (e.g., vertical migration, phototaxis, and gear avoidance) on sampling design. We also discuss sample preservation and processing, as well as the terminology, techniques, and taxonomic guides used in the identification of zooplankton and fish eggs and larvae.
Early investigators studying the growth, reproduction, and mortality of fish populations documented the critical importance of early life stages to overall abundance (Hjort 1914). Fishes are relatively fecund vertebrates, but most populations exhibit high egg and larval mortality (>90%) and significant year-to-year variation in early life stage survival (Nash and Dickey-Collas 2005; Straile et al. 2007) that can ultimately influence recruitment to adult stocks (Beaugrand et al. 2003; Head et al. 2005). Ichthyoplankton mortality is usually attributed to inherited defects, egg quality, starvation, disease, predation, and environmental fluctuations (Pepin et al. 2002; North et al. 2005; Zeldis et al. 2005). Periods of high mortality are often associated with critical events in early ontogeny (e.g., hatching, first feeding, or initiation of swim bladder function; Nislow et al. 2004; Armstrong and Nislow 2006), and the timing and duration of these critical periods may be closely tied to environmental variability and zooplankton prey abundance.