Seabird nestling diets reflect latitudinal temperature-dependent variation in availability of key zooplankton prey populations

Abstract

We report on historical (1978 to 1982) and more recent (1996 to 2000) variation in the nestling diet of Cassin's auklet Ptychoramphus aleuticus breeding at Triangle Island (southern) and Frederick Island (northern), British Columbia, Canada; these islands are influenced by the California and the Alaska Current ecosystems, respectively. Ocean climate conditions off the British Columbia coast varied tremendously between 1978 and 2000. At both colonies, the nestling diet was composed largely of copepods and euphausiids, with fish contributing substantially in some of the warmer years at Triangle Island. The copepod Neocalanus cristatus was the single most important prey item at both colonies, and Stage V copepodites dominated in all sampling periods. We used a recently published temperature-dependent phenology equation to estimate the timing of peak biomass of Neocalanus near Triangle and Frederick Islands. During warm water years (such as 1996 and the El Niño of 1998), the timing and duration of N. cristatus availability in surface waters near Triangle Island was early and limited (mismatched) in contrast to cooler years (such as 1999 and 2000), when this prey was available to birds throughout the breeding season (matched). We argue that Cassin's auklet nestling diet data reflect the temperature-related timing of Neocalanus prey availability to seabirds in surface waters. Our results support the argument that inadequate overlap of prey availability and predator breeding (i.e. temporal trophic mismatch) is more likely on Triangle Island, where zooplankton peaks often occur earlier and are narrower, than on Frederick Island, where prey peaks are later and more protracted. Poor reproductive performance is the biological consequence of such trophic mismatch for Cassin's auklet. If the frequency of El Niño-like events increases and if ocean temperatures rise in the future, we predict an increase in the frequency of trophic mismatch events in the northeast Pacific Ocean. KEY WORDS:Copepod availability · Ocean temperature · Match-mismatch · Seabirds · Cassin's auklet Resale or republication not permitted without written consent of the publisher Contribution to the Theme Section 'Marine ecosystems, climate and phenology: impacts on top predators'OPEN PEN

Latitudinal temperature-dependent variation in timing of prey availability can impact Pacific seabird populations in Canada

“…Research on seabird nestling diet is consistent with the concept that the phenology of temperature-dependent copepod prey and Cassin's Auklet predator populations can be mismatched on Triangle Island in the California Current Ecosystem but a similar mismatch has not been observed on Frederick Island, in the Gulf of Alaska Ecosystem (Bertram et al 2009). In addition, local adult annual survival for Cassin's Auklet is lower on Triangle Island, where the population has declined significantly (1989-2009, Rodway and Lemon 2011), than on Frederick Island (Bertram et al 2005) where declines have not been reported.…”

“…as an index for the timing for N. cristatus in response to interannual variation in SST near the seabird colonies. Note that N. cristatus represented > 99% of the copepod biomass in the nestling diet of Cassin's Auklet on Triangle and Frederick islands during the time frame of this study (Bertram et al 2009). Note, too, that Batten and Mackas (2009) demonstrated a shortened duration of the annual N. plumchrus biomass peak in the Northeast Pacific in response to ocean climate warming, in addition to earlier peak biomass timing.…”

“…Monthly SST from Pine Island (50°35'N, 127°26'W) was used to estimate timing of prey biomass at Triangle Island, and SST data from Langara Island (54°09'N, 133°02'W) light station was used for Frederick Island. Pine Island light station SST data have been used previously (Bertram et al 2001(Bertram et al , 2009 and provide a valid index of SST within foraging ranges (Boyd et al 2008) relative to a 6 degree baseline was calculated as (SST°C-6°C)*31d for March and May or (SST°C-6°C)*30d for April (Mackas et al 2007). The sum of the degree days for these three months (x) was then used in a temperature-dependent phenology equation to obtain point estimates for the timing of peak biomass of N. plumchrus ((Marukawa, 1921)) near Triangle Island and Frederick Island (date of peak biomass= 169 -0.16x; Mackas et al 2007).…”

“…If future ocean climate warming continues to push the centre of zooplankton abundance poleward (Mackas et al 2007) and to induce earlier and more narrow timing of Neocalanus availability (Batten and Mackas 2009), we expect that episodes of poor nestling growth will increase in frequency on Triangle Island and could eventually affect auklets on more northerly colonies like Frederick Island. We argue that the seabird colonies on Triangle and Frederick islands, because they reflect variation in distinct large marine ecosystems at different latitudes, represent key locations to gauge reproduction and survival that are needed to identify the causes of population trends for upper trophic level predators in the Northeast Pacific (see also Bertram et al 2005Bertram et al , 2009.…”

Estimation of Coast-Wide Population Trends of Marbled Murrelets in Canada Using a Bayesian Hierarchical Model

“…In 2005, an unusual atmospheric blocking event severely delayed spring transition winds and hence marine production in the California Current Ecosystem [ 37 ]. Colonial seabirds off the West Coast of Washington [ 38 ] and Vancouver Island [ 39 , 40 ] failed to breed in those two years. Thus, concurrent movements of marbled murrelets away from marine areas that lack prey could explain the interannual variation in radar counts.…”