Guest essay by Eric Worrall
A new study claims that Scottish sea birds are being forced to diversify their diet by a shortage of their favourite prey species. But in my opinion the study ignores other issues, such as chemical pollution and long term radioactive contamination in regions adjacent to the study location.
Warmer water signals change for Scotland’s shags
Three decades of data from the Isle of May, off Scotland’s east coast, found that the proportion of sandeels – the bird’s usual fayre – declined by 48% between 1985 and 2014. Over the same period, the number of other fish prey in the diet increased, from an average of just one species per year in 1985 to eleven in 2014.
The North Sea is one of the most rapidly warming marine ecosystems on the planet, and warmed by 0.037 degrees Celsius per year between 1982 and 2012. Lead author, Richard Howells, explained that the study, “ties in with many observations of changes in the abundance, distribution and phenology of many species in the North Sea, and a decline in the availability and size of sandeels.
“Climate models predict further increases in sea surface temperature and weather variability in the region, with generalisation in shag diet, one way in which this species appears to be responding to this change.”
Short-term weather conditions also impacted on the bird’s ability to feed, with “windier conditions on a daily basis linked to fewer sandeel in the diet. This may affect the ability of parents to successfully feed their chicks,” Howells said.
”Changes in the prey types consumed by this population suggest that adults may now be hunting across a broader range of habitats than they did in the past, such as rocky habitats where they can find the Rock Butterfish. Such changes may alter interactions with potential threats, such as small-scale offshore renewable developments.”
The abstract of the study;
From days to decades: short- and long-term variation in environmental conditions affect offspring diet composition of a marine top predator
Richard J. Howells, Sarah J. Burthe, Jon A. Green, Michael P. Harris, Mark A. Newell, Adam Butler,
David G. Johns, Edward J. Carnell, Sarah Wanless, Francis Daunt
ABSTRACT: Long-term changes in climate are affecting the abundance, distribution and phenology of species across all trophic levels. Short-term climate variability is also having a profound impact on species and trophic interactions. Crucially, species will experience long- and short-term variation simultaneously, and both are predicted to change, yet studies tend to focus on only one of these temporal scales. Apex predators are sensitive to long-term climate-driven changes in prey populations and short-term effects of weather on prey availability, both of which could result in changes of diet. We investigated temporal trends and effects of long- and short-term environmental variability on chick diet composition in a North Sea population of European shags Phalacrocorax aristotelis between 1985 and 2014. The proportion of their principal prey, lesser sandeel Ammodytes marinus, declined from 0.99 (1985) to 0.51 (2014), and estimated sandeel size declined from 104.5 to 92.0 mm. Concurrently, diet diversification increased from 1.32 (1985) to 11.05 (2014) prey types yr-1, including members of the families Pholidae, Callionymidae and Gadidae. The relative proportion of adult to juvenile sandeel was greater following low sea surface temperatures (SSTs) in the previous year. In contrast, the proportion of Pholidae and prey richness were higher following high SST in the previous year. Within a season, the proportion of sandeel in the diet was lower on days with higher wind speeds. Crucially, our results showed that diet diversification was linked to trends in SST?. Thus, predicted changes in climate means and variability may have important implications for diet composition of European shags in the future, with potential consequences for population dynamics.
My biggest issue with this study is that the authors are calling climate, based on data from just one location.
But delving deeper, there is another problem;
Annual sandeel proxies
As there are no long-term abundance data for the local sandeel population upon which Isle of May shags feed, we utilised 3 environmental proxies of sandeel availability.
Sea Surface Temperature (SST): SST affects sand-eel recruitment via the bottom-up effects of temperature on the availability of key copepod prey (Wright & Bailey 1993, Arnott & Ruxton 2002, van Deurs et al. 2009). Monthly SST data were obtained from the German ‘Bundesamt fur Seeschifffart und Hydro- graphie’ (www.bsh.de). Following Frederiksen et al. (2004), we calculated the mean of February and March SST for an inshore area surrounding the Isle of May (bounded by ca. 56°0’ to 56°4’N, and 2°7’ to 2° 3’ W), overlapping with the summer foraging range of this population (Bogdanova et al. 2014).
Calanus abundance: calanoid copepods, in particular the eggs and nauplii of Calanus finmarchicus, are of key importance to survival probability of early life stages of sandeels (Macer 1966, Arnott & Ruxton 2002, van Deurs et al. 2009). We analysed 1597 samples from the continuous plankton recorder (see Reid et al. 2003 for an overview) taken from a bounding box surrounding the Isle of May (55° to 58° N, 3° 0° E), between 1984 and 2014. This box is larger than the summer foraging range of the study population, but ensured there were sufficient data for the analysis. We included 2 measures of Calanus: C. finmarchicus (stages V to VI) abundance (as a proxy for C. finmarchicus egg production; van Deurs et al. 2009) and Calanus nauplii abundance (for all species combined, as species-specific abundances were unavailable). For each measure, we calculated mean monthly abundance from February to May, since these months constitute the principal period of larval sandeel feeding (Wright & Bailey 1996, van Deurs et al. 2009).
Lagged covariates: the abundance of 1+ group sandeels is dependent on conditions experienced as 0 group fish in the previous year (Arnott & Ruxton 2002). We therefore considered SST, C. finmarchicus (stages V to VI) abundance and Calanus nauplii abundance lagged by 1 yr as indices of the abundance of 1+ sandeel in the current year.
Read more (full text, p230-231): http://www.int-res.com/articles/meps_oa/m583p227.pdf
The researchers didn’t have a direct measure of the availability of sand-eels – they used proxies, including sea surface temperature, to estimate the local sand-eel population. They then concluded that sea surface temperature was the key issue.
The Firth of Forth has seen several serious pollution scandals.
Back in 2007, a pump breakdown caused 170,000 tons of raw sewage to discharge into the estuary. The Firth of Forth also suffers substantial industrial pollution, including substantial levels of resinous plastics pre-cursors.
In 2014, Dalgety Bay on the shores of the Firth of Forth, just over the water from Edinburgh, was subject to a radiation scare – the beach was closed to the public, so it could be decontaminated by the Ministry of Defence.
I don’t know how much impact all this contamination has on the local wildlife. There is a lot of water in Firth of Forth estuary to dilute all that pollution. The radioactive contamination might have been mild – it doesn’t take much radiation to get bureaucrats excited.
On the other hand, the sea birds are at the top of a long food chain which stretches throughout the estuary. Apex predators and their immediate prey are often the most at risk of biomagnification of toxins through the food chain.
In my opinion, given the limited scope of the study – just one location – the indirect measurement of a key factor (population of sand eels), and the risk that other factors such as long term contamination with radioactive waste and other pollutants could have driven changes in diet, it seems premature to suggest climate change is the main culprit, without performing further studies in other hopefully less contaminated locations.