Diet, activity budgets, and movement patterns of harbour seals (Phoca vitulina) in Cowichan Bay and adjacent areas
Date
1995
Authors
Cottrell, Paul Edward
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Abstract
The diet, activity budgets, and movements of harbour seal s are important for understanding the species interactions with fis heries, but they are poorly documented and have been studied separately from one another. Harbour seals are considered opportunistic predators that consume solitary and schooling fishes and cephalopods of most sizes and species. However, this view is based upon isolated studies on diet and movement studies that have strong biases due to the methods employed.
This project investigated harbour seals using Cowichan Bay and adjacent areas, and the interaction of seasonal changes in diet with movement patterns, population fluctuations, and activity budgets. Seasonal and diel variation in prey distribution, abundance, and behaviour also were examined in relation to harbour seal diet, movement patterns, and diel haul-out and swimming patterns.
Captive harbour seal feeding studies were conducted to evaluate and improve techniques for prey identification. Teleost otoliths in faeces have been used to characterize pinniped diet. I evaluated the potential of using hard parts other than otoliths for identifying prey. Four harbour seals were fed five fish species during experiments. Otoliths represented only 17% of >22,000 hard parts identified, while vertebrae represented 66% (excluding scales). Incorporating all prey hard parts into prey identification decreased the likelihood of not identifying prey or incorrectly identifying prey.
A total of 471 prey occurrences (x=l.5 per scat) representing 19 species in 16 families were identified. Teleost fish represented 93% of all prey identified. Prey diversity was greatest during September and October. Seasonal variation in the diet was pronounced for most prey species. This variation appeared to be a result of annual changes in prey abundance, distribution, or behaviour.
Nineteen harbour seals were captured and fitted with radio-tags. Seals were monitored every 3-5 minutes for 20 seconds from May 1991 to June 1992 from a remote monitoring station located in Cowichan Bay. The diel and seasonal haulout and swimming patterns of radio-tagged seals were determined. In addition, the movement patterns and foraging areas of radio-tagged seals outside the study area were located by finding animals from land or boat with a hand held radio-receiver.
Harbour seal haul-out patterns on log-booms were closely correlated with tidal height and, to a lesser extent, time of day. Sixteen of 18 seals had significant, positive linear relationships of time hauled out to tidal height. The proportion of time that seals spent hauled out was low at dawn and dusk during the summer and at dusk during the winter. The two main prey items of harbour seals in this area were herring and hake (>65%), which are diel vertical migrators. Crepuscular movements of these prey types closer to the surface combined with changes in their schooling behaviour may increase harbour seal foraging success. Seals spent more time hauled out in August to February than the rest of the year. This corresponds to the moulting period and growth of new pelage of harbour seals in the area.
The diet, activity budgets, and movements of harbour seal s are important for understanding the species interactions with fisheries, but they are poorly documented and have been studied separately from one another. Harbour seals are considered opportunistic predators that consume solitary and schooling fishes and cephalopods of most sizes and species. However, this view is based upon isolated studies on diet and movement studies that have strong biases due to the methods employed. This project investigated harbour seals using Cowichan Bay and adjacent areas, and the interaction of seasonal changes in diet with movement patterns, population fluctuations, and activity budgets. Seasonal and diel variation in prey distribution, abundance, and behaviour also were examined in relation to harbour seal diet, movement patterns, and diel haul-out and swimming patterns. Captive harbour seal feeding studies were conducted to evaluate and improve techniques for prey identification. Teleost otoliths in faeces have been used to characterize pinniped diet. I evaluated the potential of using hard parts other than otoliths for identifying prey. Four harbour seals were fed five fish species during experiments. Otoliths represented only 17% of >22,000 hard parts identified, while vertebrae represented 66% (excluding scales). Incorporating all prey hard parts into prey identification decreased the likelihood of not identifying prey or incorrectly identifying prey. A total of 471 prey occurrences (x=l.5 per scat) representing 19 species in 16 families were identified. Teleost fish represented 93% of all prey identified. Prey diversity was greatest during September and October. Seasonal variation in the diet was pronounced for most prey species. This variation appeared to be a result of annual changes in prey abundance, distribution, or behaviour. Nineteen harbour seals were captured and fitted with radio-tags. Seals were monitored every 3-5 minutes for 20 seconds from May 1991 to June 1992 from a remote monitoring station located in Cowichan Bay. The diel and seasonal haulout and swimming patterns of radio-tagged seals were determined. In addition, the movement patterns and foraging areas of radio-tagged seals outside the study area were located by finding animals from land or boat with a hand held radio-receiver. Harbour seal haul-out patterns on log-booms were closely correlated with tidal height and, to a lesser extent, time of day. Sixteen of 18 seals had significant, positive linear relationships of time hauled out to tidal height. The proportion of time that seals spent hauled out was low at dawn and dusk during the summer and at dusk during the winter. The two main prey items of harbour seals in this area were herring and hake (>65%), which are diel vertical migrators. Crepuscular movements of these prey types closer to the surface combined with changes in their schooling behaviour may increase harbour seal foraging success. Seals spent more time hauled out in August to February than the rest of the year. This corresponds to the moulting period and growth of new pelage of harbour seals in the area.
The diet, activity budgets, and movements of harbour seal s are important for understanding the species interactions with fisheries, but they are poorly documented and have been studied separately from one another. Harbour seals are considered opportunistic predators that consume solitary and schooling fishes and cephalopods of most sizes and species. However, this view is based upon isolated studies on diet and movement studies that have strong biases due to the methods employed. This project investigated harbour seals using Cowichan Bay and adjacent areas, and the interaction of seasonal changes in diet with movement patterns, population fluctuations, and activity budgets. Seasonal and diel variation in prey distribution, abundance, and behaviour also were examined in relation to harbour seal diet, movement patterns, and diel haul-out and swimming patterns. Captive harbour seal feeding studies were conducted to evaluate and improve techniques for prey identification. Teleost otoliths in faeces have been used to characterize pinniped diet. I evaluated the potential of using hard parts other than otoliths for identifying prey. Four harbour seals were fed five fish species during experiments. Otoliths represented only 17% of >22,000 hard parts identified, while vertebrae represented 66% (excluding scales). Incorporating all prey hard parts into prey identification decreased the likelihood of not identifying prey or incorrectly identifying prey. A total of 471 prey occurrences (x=l.5 per scat) representing 19 species in 16 families were identified. Teleost fish represented 93% of all prey identified. Prey diversity was greatest during September and October. Seasonal variation in the diet was pronounced for most prey species. This variation appeared to be a result of annual changes in prey abundance, distribution, or behaviour. Nineteen harbour seals were captured and fitted with radio-tags. Seals were monitored every 3-5 minutes for 20 seconds from May 1991 to June 1992 from a remote monitoring station located in Cowichan Bay. The diel and seasonal haulout and swimming patterns of radio-tagged seals were determined. In addition, the movement patterns and foraging areas of radio-tagged seals outside the study area were located by finding animals from land or boat with a hand held radio-receiver. Harbour seal haul-out patterns on log-booms were closely correlated with tidal height and, to a lesser extent, time of day. Sixteen of 18 seals had significant, positive linear relationships of time hauled out to tidal height. The proportion of time that seals spent hauled out was low at dawn and dusk during the summer and at dusk during the winter. The two main prey items of harbour seals in this area were herring and hake (>65%), which are diel vertical migrators. Crepuscular movements of these prey types closer to the surface combined with changes in their schooling behaviour may increase harbour seal foraging success. Seals spent more time hauled out in August to February than the rest of the year. This corresponds to the moulting period and growth of new pelage of harbour seals in the area.