Biologging data from foraging harbour seals shows less impact on outmigrating salmon than expected. A few seals in the study population targetted juvenile coho, and exerted less pressure on chinook—appearing instead to target larger fish preying on juvenile chinook

Predation by harbour seals is believed to significantly impact juvenile coho and Chinook salmon as they enter the ocean — and are thought by some to be responsible for the poor return of spawning adults. Researchers from the University of British Columbia set out to determine who was eating juvenile salmon, and when and where it was occurring by capturing and tracking harbour seals that carried cell-phone-like devices that recorded everything and everywhere the seals went.   
Harbour Seal MMRU

Harbour seals tend to haulout to rest as the tide falls and intertidal areas are exposed. 

In a recently published paper in the journal, Marine Ecology Progress Series, Hassen Allegue and colleagues report that only a few of the seals they followed targetted outmigrating coho salmon. They were equally surprised when they discovered that the seals did not seem to take the smaller-bodied chinook, but appeared instead to target fish preying on juvenile chinook. Most of the feeding-type behaviours occurred as tide height rose (which were higher at dusk and night), especially during full moonlight. 
Harbour Seal 2 MMRU

Resting harbour seals showing one of the study animals wearing the tracking device.

These findings bring new insight into the complexity of the interactions between harbour seals and out-migrating coho and Chinook smolts. As the authors note, “they show the highly individualistic nature of seal predation, and suggest that there may be complex indirect effects associated with seals consuming fish that also prey on juvenile salmon.”  “Such confounding factors need to be addressed when considering broad conservation measures that might be taken against harbour seals to enhance the marine survival of salmonids.”

Publications PUBLICATION


2020
 
Harbour seals responded differently to pulses of out-migrating coho and Chinook smolts.
Allegue, H., A.C. Thomas, Y. Liu, and A.W. Trites. 2020.
Marine Ecology Progress Series 647:21-227.
abstract
There is increasing evidence that predation by harbour seals on out-migrating salmon smolts may be responsible for the low return of adult coho and Chinook salmon in the Salish Sea. However, little attention has been given to understanding where and when this predation occurs, and the extent to which it might be conducted by few or many seals in the population. We equipped 17 harbour seals with data-loggers to track seal movements, and used accelerometry to infer prey encounter events (PEE) following the release of ~384,000 coho (May 4th) and ~3 million Chinook smolts (May 14th) into the Big Qualicum River. We found a small proportion (5.7%) of all PEE occurred in the estuary where salmon smolts entered the ocean-and that only one-quarter of the seals actively fed there. PEE counts increased in the estuary after both species of smolts were released. However, the response of the seals was less synchronous and occurred over a greater range of depths following the release of the smaller-bodied and more abundant Chinook smolts. Harbour seals feeding in the estuary appeared to target coho smolts at the beginning of May, but appeared to pursue predators of Chinook smolts in mid-May. PEE counts in the estuary increased as tide height rose, and were higher at dusk and night-especially during full moonlight. Such fine-scale behavioural information about harbour seals in relation to pulses of out-migrating smolts can be used to design mitigation strategies to reduce predation pressure by seals on salmon populations.

keywords     predation, salmon, harbour seals, coho, Chinook, data-loggers, biologging, accelerometry, accelerometers, prey encounters, estuary, size selection, mitigation, PIT tags
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