Research Document - 2011/018
Seasonal patterns in the spatial overlap of southern Gulf of St. Lawrence cod and grey seals, with a discussion of sources of error and possible bias
By H.P. Benoît, D.P. Swain, and M.O. Hammill
Abstract
A Zonal Assessment Process on the potential impacts of grey seals on fish populations in eastern Canada was held during October 4-8, 2010. The terms of reference for that meeting included assessing the temporal and spatial overlap of grey seals and cod, as well as the possible sources of bias and uncertainty in estimates of the seasonal distribution of both species. This research document was prepared to address these questions as it relates to the southern Gulf of St. Lawrence (sGSL) cod population and to grey seals in the sGSL.
The spatial overlap of two size categories (<35 cm and ≥35 cm) of sGSL cod and grey seals was examined on a monthly timescale and a spatial scale of 1000s of km2. The distribution of cod was inferred from seasonal bottom-trawl surveys and seasonal changes in the spatial distribution of cod-directed commercial fishing effort. The distribution of grey seals was inferred from the tracking of satellite-tagged individuals. There were strong herd, gender and stage-specific differences in grey seal movements. Juvenile and adult male grey seals from the Gulf herd overlapped most with southern Gulf cod, with a mean monthly average of 16% of the herd overlapping with cod. Overlap with seals from Sable Island was smaller (<1%).
Movement patterns of seals are inferred from a very small number of tagged seals, particularly given the need to account for gender and stage-specific differences in movements. Because of this low sample size, areas that are visited by only a small proportion of the herd have a high probability of being underrepresented in the inferred distributional areas, particularly when a fairly fine spatial resolution is used. To the extent that these are areas where cod occur, seal-cod overlap will be under or over-estimated. For periods when cod are aggregated (winter, spawning, migration) in particular, potential consumption of cod by seals may therefore also be underestimated. Unfortunately, the sample size of tagged seals is especially low during some of these periods. There are also likely biases in seal tagging with respect to haul-out site that may affect population-level inferences on grey seal distribution and movement.
The current spatial distribution of cod in months other than August and September can only be inferred from surveys that took place 15-20 years ago. The distribution is poorly known during some months, such as during cod migration. Furthermore, our characterization of monthly changes in distribution may not be entirely accurate given changes in the timing of spring and fall migrations.
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