Sea temperature variations mediate annual changes in the diet of Australian fur seals in Bass Strait

Abstract

Using a 9-yr data set, we investigated annual fluctuations in the diet of an apex predator, the Australian fur seal Arctocephalus pusillus doriferus. At Seal Rocks (northern Bass Strait), home to 25% of the entire species population, we assessed diet through collections (1997 to 2006) of scat and regurgitate samples. We identified prey remains of 42 fish taxa and 7 cephalopod taxa. Only crustaceans that were fish parasites or fish prey (amphipods and isopods) were found; no birds were identified in the samples. Six species represented 80% (as frequency of occurrence) of the fish prey, and the arrow squid Nototodarus gouldi represented 70% of cephalopod prey. There was significant annual variability in the diet. Principal component analysis indicated this was variability due to the presence of redbait Emmelichthys nitidus in some years, and its near absence and replacement in other years by increased proportions of barracouta Thyrsites atun, red cod Pseudophycis bachus and leatherjackets (Family Triglidae). Generalised Linear Models indicated the annual variation was related to mean sea surface temperatures in western Bass Strait where the seals foraged. Redbait proliferated in cooler years and were less abundant in warmer years. No corresponding annual correlation was evident between the prey assemblages and either annual fisheries catch-per-unit-effort or the annual mean Southern Oscillation Index. The propensity for diet regimes to persist for several years, then change suggests that oceanographic fluctuations probably influence previously unrecognised multi-year cyclic fluctuations of prey and of Bass Strait ecosystems. KEY WORDS: Arctocephalus · Fur seal diet · Sea temperature · Bass strait · Emmelicthys nitidus Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 369: [297][298][299][300][301][302][303][304][305][306][307][308][309] 2008 Arnould & Kirkwood 2008). Like other fur seals, such as the New Zealand fur seal A. forsteri (Harcourt et al. 2002), Australian fur seals are generalist predators and consume a range of benthic and pelagic prey; they exhibit some spatial and temporal variations in diet. During the 1920s and 1940s, their diet at breeding colonies in northern Bass Strait was determined by inspection of stomach contents in several shot animals, and comprised mainly barracouta (also known as snoek Thyrsites atun), cephalopods, and southern rock lobster Jasus novaehollandiae (Lewis 1930, McNally & Lynch 1954. In the 1980s and 1990s, the diet, determined from prey hard parts in scats and regurgitates at sites in southern Bass Strait and south of Tasmania, comprised mainly redbait Emmelichthys nitidus, jack mackerel Trachurus declivis, leatherjackets (Family Monocanthidae) and arrow squid Nototodarus gouldi (Gales et al. 1993, Gales & Pemberton 1994, Hume et al. 2004. Between 1997 and 2001 in northeastern Bass Strait, the same prey, plus red cod Pseudophycis bachus were the key species (Littnan 2003, Littnan et al. 2007; during 2001/2002 in ...

Epizootiology of Brucella Infection in Australian Fur Seals

“…In northern Bass Strait, interannual fluctuations in sea-surface temperature have been correlated with broad fluctuations in the prey composition of Australian fur seals (Kirkwood et al, 2008). The increase in Brucella prevalence between 2003 and 2005 corresponded with a protracted period of cooler-thanaverage sea surface temperatures and higher-than-average proportions of the fish redbait (Emmelichthys nitidus) in the diet of Australian fur seals in northern Bass Strait.…”

“…The pupping season of Australian fur seals is synchronized and occurs between mid-November and mid-December (Gibbens and Arnould, 2009 To investigate possible spatial differences in antibody prevalence, pups from three other Australian fur seal colonies, The Skerries (37u459S, 149u319E), Seal Rocks (38u309S, 145u109E) and Lady Julia Percy Island (38u429S, 142u009E), were sampled between late December 2007 and late January 2008. This sampling was conducted concurrently with a mark-recapture study (Kirkwood et al, 2010).…”

“…WinPepi epidemiologic software, Version 10.5 (Abramson, 2004), was utilized. The estimated size of the Kanowna Island colony is 15,000 individuals, which includes an annual production of approximately 3,400 pups (Kirkwood et al, 2010). A desired sample size of 59 for each age class was required to enable a 95% probability of detecting at least one positive animal if antibody prevalence was 5% or greater.…”

Stable isotopes reveal inter-annual and inter-individual variation in the diet of female Australian fur seals

“…redbait Emmelichthys nitidus, barracouta Thyrsites atun, jack mackerel Trachurus declivis, red cod Pseudophysis bachus and leatherjacket (family Monocanthidae), as well as of Gould's squid Nototodarus gouldi (Gales et al 1993, Gales & Pemberton 1994, Hume et al 2004, Page et al 2005, Littnan et al 2007, Kirkwood et al 2008. These studies, however, have all demonstrated that Australian fur seals regularly consume a wide variety of prey (43 to 49 species) and that the species is a generalist predator (Littnan et al 2007).…”

“…From analysis of faecal samples collected over a 9 yr period, Kirkwood et al (2008) documented a major inter-annual shift in Australian fur seal diet composition. A multi-year increase in the frequency of occurrence of redbait coincided with decreases in red cod and barracouta, as well as a reduction in the size of Gould's squid.…”