Feeding ecology of larval Atlantic herring (Clupea harengus): linking main prey availability and recruitment in Trinity Bay, Newfoundland

Wilson, Carissa Josephine (2016) Feeding ecology of larval Atlantic herring (Clupea harengus): linking main prey availability and recruitment in Trinity Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Newfoundland Atlantic herring (Clupea harengus) populations are composed of spring- and autumn- spawning components, targeted as a mixed fishery. Until the early 2000s, the spring-spawning component accounted for ~90% of the total catch. Within the last decade, the relative abundance of spring-spawning herring has decreased and autumn-spawning herring now dominate the catch in most areas. Year-class strength is largely determined by survival rate during the larval stage. The objective of this study is to identify the main prey of larval herring and explore the link between main prey availability and subsequent recruitment strength. Herring larvae were collected using bongo tows in Trinity Bay, Newfoundland, in the autumns of 2002 (15-52 mm Standard Length), 2006 (5-15 mm SL) and 2013 (5-15 mm SL). Diet composition was identified and otoliths were extracted to estimate age. Nauplius stages of the calanoid copepod Temora longicornis and the cyclopoid copepod Oithona similis dominated the diet during the early larval stage (5-15 mm SL) in 2006 and 2013, respectively. In 2002, the mid-size calanoid copepod Pseudocalanus sp. strongly dominated the diet in the mid-larval stage (15-30 mm SL). In the late larval stage (>30 mm SL) in 2002, larval diet showed a shift to the larger calanoid copepod Calanus sp. The seasonal abundance peak of the main prey during the mid-larval stage, Pseudocalanus sp., shifted from spring to autumn during the mid-2000s, concurrent with the period of changing relative abundance of spawning components. A positive relationship was found between the abundance of Pseudocalanus sp. in October and recruitment of autumn-spawning herring, thereby supporting the idea that survival may be driven by preferred prey availability during the larval stage. We could not reject the hypothesis that the observed shift in herring population dynamics in the mid-2000s resulted from higher survival rates in autumn-hatched larvae through increased availability of their main prey.

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