Spatiotemporal variability in fatty acid profiles of the copepod Calanus marshallae off the west coast of Vancouver Island




Bevan, Daniel

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Factors affecting energy transfer to higher trophic levels can determine the survival and production of commercially important species and thus the success of fisheries management regimes. Juvenile salmon experience particularly high mortality during their early marine residence, but the root causes of this mortality remain uncertain. One potential contributing factor is the food quality encountered at this critical time. The nutritionally vital essential fatty acids (EFA) docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3) are essential to all marine heterotrophs, and their availability has the potential to affect energy transfer through a limitation-driven food quality effect. Assessing variability in DHA and EPA in an ecologically important prey species of juvenile salmon could give insight into the prevalence and severity of food quality effects. On the west coast of Vancouver Island (WCVI), one such species is the calanoid copepod Calanus marshallae. This omnivorous species possesses a high grazing capacity and the ability to store large amounts of lipids. As it is also an important prey item for a diverse array of predators, including juvenile Pacific salmon, C. marshallae plays a key role in energy transfer from phytoplankton to high-trophic iv consumers. This study quantified spatiotemporal variability in the quality of C. marshallae as prey for higher trophic levels using three polyunsaturated fatty acid indicators: DHA:EPA, %EFA and PUFA:SFA (polyunsaturated fatty acids to saturated fatty acids). Samples were collected on the WCVI in May and September of 2010 and May 2011. The environmental parameters included in the analysis were the phase of the Pacific Decadal Oscillation (PDO), sea surface temperature (SST), latitude, station depth, and season (spring versus late summer). Despite a phase shift in the PDO from positive to negative, overall means of the fatty acid indicators did not vary between May 2010 and May 2011. Same-station %EFA values rarely fluctuated more than 5%. DHA:EPA ratios were more variable but without a discernable pattern, while PUFA:SFA ratios decreased in shelf stations and increased offshore. Contrary to expectations, fatty acid indicators showed a weak positive correlation or no relationship with SST, nor was there a relationship with latitude. The narrow temperature range observed across all stations suggests that temperature may not play a significant role in PUFA availability off the WCVI. There were, however, significant relationships between the fatty acid indicators and bottom depth and season. Shelf and slope stations showed significantly higher %EFA and PUFA:SFA than did offshore stations (depth >800 m), with this gradient appearing stronger in May than September. While the food quality represented by C. marshallae was consistent across all shelf stations, the lower food quality observed offshore could potentially affect juvenile salmon growth along the WCVI where the shelf narrows to less than 5 km.



Polyunsaturated fatty acids, DHA, EPA, Copepod, Juvenile salmon, Food quality, Calanus marshallae