Warmer water worsens fish nutritional quality - study

Warmer water events can worsen the nutritional balance of fish, finds new research carried out in New Zealand.

Conversely, the new study, just published in the Journal of Animal Ecology, found fish and squid caught when waters were colder than normal were of higher quality.

Its lead author says the findings have implications for a range of species that eat fish - including us - and highlight another potential knock-on effect of climate change.

Led by the University of Sydney, the research drew on a highly successful marine predator seabird - the Australasian gannet - as a biological monitor of the marine environment and food sources.

The team combined miniature bird-borne GPS loggers, fish and squid nutritional analysis and nutritional modelling, and quantified colder and warmer water events by comparing the mean sea surface temperature with 10 years of data.

Fish and squid captured by gannets were found to have significantly lower ratio of healthy oils to protein during warm water events - where sea surface temperature (SST) was warmer than the 10-year mean - and better nutritional quality during cold water periods, or where SSTs were lower than the 10-year mean.

"Marine mammals and seabirds such as gannets eat similar foods as humans - namely fish and squid," said lead author Dr Gabriel Machovsky-Capuska.

"All capture prey in similar areas, and inevitably all are impacted by nutritional changes to this food source."

Co-author Professor David Raubenheimer, also of the University of Sydney, said the research team had devised a novel approach to gain the insights.

"Our approach, which we call nutritional landscapes, allows us to associate the nutritional quality of marine resources - otherwise very challenging, as marine life continuously moves - with geographic location, water depth and environmental conditions such as sea surface temperature and chlorophyll levels," Raubenheimer said.

"These findings underline the importance of linking marine environmental fluctuations with the nutritional quality of fish and squid for human consumption - and provide significant insights for fisheries that are capturing fish for humans to eat."

Machovsky-Capuska said the findings were also revealing for environmental and conservation purposes.

"The work shows that diet and foraging behaviour of marine predators are significantly influenced by warm and cold events," he said.

"During warm-water events gannets had to work harder for their food as they expanded their foraging habitat and increased their foraging trip duration, while at the same time consuming prey and diets with lower content of energy-providing oils.

"Our approach can be used to understand and ultimately protect travelling routes for migratory species, and could support the conservation of endangered species in terms of food quality and habitat suitability."

Machovsky-Capuska said the study also offered another potential sentinel of climate change's impacts on our oceans.

It followed a dramatic marine heatwave around New Zealand over summer, which pushed SSTs to a record 1.5C above average and saw tropical fish from Australia enter our waters.

While the event has been put down to a freak combination of drivers, recent research has shown marine heatwaves had become longer, stronger and more frequent over the past century, and the trend would continue as the world warmed.

The latest study was carried out in collaboration with researchers from Queensland's James Cook University, Massey University and the Ornithological Society of New Zealand.