Tests of larval retention in a tidally energetic environment reveal the complexity of the spatial structure in herring populations

Dispersion during the larval phase is of central importance in the dynamics of marine fish and invertebrate populations. Rapid transport or dispersion of larvae may contribute to connectivity and mixing, whereas spatial persistence (retention) is hypothesized to favour stock complexity and local subpopulations. Larval retention, while rarely quantified, may be defined in species with protracted spawning by the spatial co-occurrence of larvae of different sizes or ages. The spatial distributions of larval Atlantic herring (Clupea harengus) were examined from 22 annual autumn surveys (1975-1998) and 9 spring surveys (1975-1984) from the Bay of Fundy, a region with large tides and residual flow. Larvae of all sizes (3-27mm in length, from hatch to nearly 4months post-hatch) were observed each year in two major aggregations; one off southwestern Nova Scotia, and the other in the mid-inner Bay of Fundy off the northwestern shore of Nova Scotia. Two similar aggregations were evident over 5months later from 9 spring surveys (1975-1984), despite the residual flow that would have swept the larvae from the region within 1month. Larval retention was apparent from overlapping centres of mass of different size (=age) classes of larvae, and tested using a size diversity index, based on the co-occurrence of 1-mm-size categories, derived from protracted spawning of several weeks. Geospatial hot spots' (G(i)* statistic) of four size (age) classes were evident at specific stations in the 50-100m bathymetric zone and not elsewhere. These metrics provide quantitative measures of retention that may be applied to many ichthyoplankton data sets. One of the three main spawning areas collapsed during the study period after a period of intense fishing and failed to rebuild, but there was no substantial change in the location of larval hotspots in subsequent years. While larval retention does not directly relate to each spawning location, larval retention in the Bay of Fundy contributes to the complex metapopulation' structure of herring stocks in the western Atlantic.