What’s on the menu? Diet of key Southern Ocean fish revealed

New IMAS research has shed light on the diet of small Southern Ocean fish that play an important role in the complex food web of the sub-Antarctic.

Less than 20-centrimetres in length, mesopelagic fish form a major part of fish biomass in the ocean and are a key link in the food chain between zooplankton and higher predators such as commercially fished Patagonian toothfish, seals and whales.

Despite their significance, information on the feeding habits of mesopelagic fish is sparse, particularly in the highly productive zone of the Southern Ocean around the Kerguelen Plateau, south of the Indian Ocean and midway between Australia and South Africa.

Javed Riaz dissectingA study led by IMAS PhD student Javed Riaz  and published in the journal Deep-Sea Research II analysed the stomach contents of four of the most common and widespread species of mesopelagic fish collected to south of the Kerguelen Plateau.

The researchers found a clear north-south transition in food web structure.

Krill were the dominant prey item of fish in the southern region, contrasting with assemblages to the north which primarily consume different types of crustaceans and fish.

Mr Riaz (Pictured, left, dissecting fish. Credit: Rowan Trebilco) said the importance of krill in the diet of Southern Ocean mesopelagic fish has been a major point of contention between scientists, and the study confirms these small fish play a more important role in the food web than traditionally believed.

“Our study is the first to describe the diets of key mesopelagic fish in the southern Kerguelen area using visual analysis of their stomach contents,” Mr Riaz said.

Fish sorted on research ship. Credit Rowan Trebilco“The spatial differences in the diets of mesopelagic species in this region are likely a reflection of the structure of zooplankton communities and are consistent with other studies that have found krill become increasingly prevalent in predator diets closer to the Antarctic.

(Image, right, mesopelagic fish sorting on research ship. Credit: Rowan Trebilco)

“Our results confirm the complexity and spatial variability of food webs in the Southern Ocean and improve understanding of how energy flows up the food chain in the southern Kerguelen region.

“Studies such as this are important in developing evidence-based management strategies and can help us to make informed decisions about how environmental change and harvesting might influence Southern Ocean ecosystems,” Mr Riaz said.

Authorised by the Executive Director, Institute for Marine and Antarctic Studies
February 4, 2020