Presentation Abstract

Recent studies of harbour seal diets (2012-2014) have been used to estimate the amounts of salmon consumed by seals in the Strait of Georgia. However, these diet data have primarily come from estuary habitats, and may not be representative of all seals. We analysed 1,317 scat samples collected at an estuary (Cowichan Bay) and 7 non-estuary sites from Apr–Nov 2016 and Apr–May 2017 to compare salmon consumption inside and outside of estuaries. Using high-throughput DNA techniques, we determined seals consumed a wide range of prey (n = 238 species)—with gadids (primarily hake) and forage fish (primarily herring) dominating diets in both habitats (typically >75% of diet when combined). Salmonids were consumed throughout the year. Juvenile salmonids (based on life-histories and size of recovered bones) collectively made up 1.4% (CI = 0.8–2.1%) of the spring diet at non-estuaries and 2.5% (CI = 1.4–3.9%) in Cowichan Bay in 2016/17. Primary juvenile salmon consumed were chinook, and to a lesser extent coho and chum. The 1.1% difference between sites is considerable when translated into number of smolts consumed, and indicates smolt predation was 50% higher at our estuary site. Salmon consumption spiked in the fall (driven by returning adult chum salmon), and was much higher in Cowichan Bay (35%, CI = 29–40%) than at non-estuary sites (9.1%, CI = 7.3–11.0%). Furthermore, the bulk of salmon consumed at non-estuary locations was driven by one site (Belle Chain Islets) which appeared to be heavily influenced by Fraser River runs. Our findings highlight that estuaries may not be useful as proxies for non-estuary sites when assessing predation on species of conservation concern (including salmonids) and that models estimating harbour seal related salmon mortality should consider differences in consumption rates inside and outside of estuaries in the Salish Sea.

Session Title

The Salish Sea Marine Survival Project: Bottom-up and Top-down Processes

Keywords

Harbour seal diet, Estuary, Non-estuary

Conference Track

SSE11: Species and Food Webs

Conference Name

Salish Sea Ecosystem Conference (Seattle, WA : 2018)

Document Type

Event

SSEC Identifier

SSE11-523

Start Date

6-4-2018 9:00 AM

End Date

6-4-2018 9:15 AM

Type of Presentation

Oral

Contributing Repository

Digital content made available by University Archives, Heritage Resources, Western Libraries, Western Washington University.

Geographic Coverage

Salish Sea (B.C. and Wash.)

Rights

This resource is displayed for educational purposes only and may be subject to U.S. and international copyright laws. For more information about rights or obtaining copies of this resource, please contact University Archives, Heritage Resources, Western Libraries, Western Washington University, Bellingham, WA 98225-9103, USA (360-650-7534; heritage.resources@wwu.edu) and refer to the collection name and identifier. Any materials cited must be attributed to the Salish Sea Ecosystem Conference Records, University Archives, Heritage Resources, Western Libraries, Western Washington University.

Type

text

Language

English

Format

application/pdf

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Apr 6th, 9:00 AM Apr 6th, 9:15 AM

Harbour seals consume more juvenile and adult salmon in estuaries than elsewhere in the Strait of Georgia

Recent studies of harbour seal diets (2012-2014) have been used to estimate the amounts of salmon consumed by seals in the Strait of Georgia. However, these diet data have primarily come from estuary habitats, and may not be representative of all seals. We analysed 1,317 scat samples collected at an estuary (Cowichan Bay) and 7 non-estuary sites from Apr–Nov 2016 and Apr–May 2017 to compare salmon consumption inside and outside of estuaries. Using high-throughput DNA techniques, we determined seals consumed a wide range of prey (n = 238 species)—with gadids (primarily hake) and forage fish (primarily herring) dominating diets in both habitats (typically >75% of diet when combined). Salmonids were consumed throughout the year. Juvenile salmonids (based on life-histories and size of recovered bones) collectively made up 1.4% (CI = 0.8–2.1%) of the spring diet at non-estuaries and 2.5% (CI = 1.4–3.9%) in Cowichan Bay in 2016/17. Primary juvenile salmon consumed were chinook, and to a lesser extent coho and chum. The 1.1% difference between sites is considerable when translated into number of smolts consumed, and indicates smolt predation was 50% higher at our estuary site. Salmon consumption spiked in the fall (driven by returning adult chum salmon), and was much higher in Cowichan Bay (35%, CI = 29–40%) than at non-estuary sites (9.1%, CI = 7.3–11.0%). Furthermore, the bulk of salmon consumed at non-estuary locations was driven by one site (Belle Chain Islets) which appeared to be heavily influenced by Fraser River runs. Our findings highlight that estuaries may not be useful as proxies for non-estuary sites when assessing predation on species of conservation concern (including salmonids) and that models estimating harbour seal related salmon mortality should consider differences in consumption rates inside and outside of estuaries in the Salish Sea.