Harbor seal foraging response to a seasonal resource pulse, spawning Pacific herring

Abstract

Evidence suggests that Pacific harbor seals Phoca vitulina are likely to alter their foraging behavior in response to seasonal prey pulses. We hypothesized that spawning herring Clupea pallasii aggregations are seasonally important prey for harbor seals, predicting that (1) harbor seal consumption of adult herring would peak during the spawning season, (2) harbor seals would seasonally change their foraging areas to take advantage of spawning herring aggregations, and (3) seal diving behavior would reflect the vertical distribution of herring during the spawning season. The predictions were tested using an analysis of harbor seal prey remains, GPS telemetry, and satellite-linked time/depth recorder data. Contrary to predictions, herring in harbor seal diet was comprised of 74% juveniles and 26% adults in the spawn season, versus 37% juveniles and 63% adults in the post-spawn season. Harbor seal use of documented herring areas was highest during the season when herring did not spawn, and seal diving behavior did not reflect the vertical distribution of herring. The lack of response by harbor seals to spawning herring pulses is likely explained by the low energy density of adult herring during the spawn season, and the availability of profitable alternative prey such as juvenile herring. This study highlights the influence of relative prey profitability on the foraging behavior of harbor seals, and may help to explain why predators do not always respond as predicted to resource pulses. KEY WORDS: Phoca vitulina · Resource pulse · Utilization distribution · Diet · Diving · Marine predatorsResale or republication not permitted without written consent of the publisher

Predator-prey interactions between harbor seals and migrating steelhead trout smolts revealed by acoustic telemetry

“…Uppertrophic-level predators can exert a strong influence on marine food webs, switching to less-preferred prey as preferred prey are depleted (Berriman et al 2015 and see papers cited within). Juvenile salmonids typically comprise a small component of diets of harbor seals, which feed most frequently on gadoids and clupeids in the Salish Sea (Thomas et al 2011). A major shift in the Puget Sound food web occurred in the early 1990s (Harvey et al 2012).…”

“…Harbor seals are generalist foragers, but with clear species preferences that can reflect ecosystem shifts over decadal time scales (Thomas 2015). Harbor seal diets also shift seasonally in response to fish migrations, spawning aggregations, and other factors (Olesiuk 1993, Thomas et al 2011. Steelhead smolts migrate rapidly to the Pacific Ocean and are only present in appreciable numbers in Puget Sound for about 2 mo (mid-April to mid-June), with a strong peak in abundance from early to mid-May.…”

“…Similar trends in harbor seal populations occurred in the Strait of Georgia during the same period (DFO 2010). Harbor seals feed predominantly on Clupeidae (herring) and Gadidae (cod and hake), but also feed opportunistically (Thomas et al 2011) from just a few to over 100 km from their (capture) haul-outs (Peterson et al 2012). Steelhead trout were not identified as prey in a fairly extensive year-round analysis of harbor seal diet around the San Juan Archipelago , Bromaghin et al 2013.…”

“…Conventional approaches to quantifying predator− prey interactions between pinnipeds and fish prey include a combination of spatio-temporal foraging behavior (with the use of satellite tags) and data on diet composition (Thomas et al 2011, Ward et al 2012. Small acoustic telemetry receivers (Vemco mobile transceiver; hereinafter 'VMT') capable of detecting 69 kHz transmitters (hereinafter 'tags') implanted in fish in a variety of marine species have recently been deployed on pinnipeds to test the feasibility of using pinnipeds as mobile receiver platforms (Hayes et al 2013), spatio-temporal precision, and error associated with the technique (Lidgard et al 2012), and to quantify overlap between pinnipeds and prey species (e.g.…”

A mathematical model of predator-prey interaction between seal-herring and steelhead trout

“…Their dietary seasonally alter based on mating period, prey migrations, and other factors [4,5]. When herring population decreased due to the fishing, the population of seal also reduced.…”