Understanding food webs to help manage coastal resources

A study of coastal food webs has revealed how ecosystem-based approaches to marine management could improve management of fish stocks and biodiversity in our changing coastal ocean.

Primary marine food sources, kelp and phytoplankton, have distinct ratios of carbon and nitrogen isotopes that persist throughout the food web. These isotope ratios can be used to track energy and organic matter through trophic levels in the food web. By examining these ratios in reef fish the research team from the University of Otago have shown the sources of organic matter that support fish biodiversity and how these differ between regions.

The Ecosystem Connectivity project team spent more than 700 hours underwater surveying and collecting fish species from 30 study sites during three 3-week voyages to the Marlborough Sounds and Tasman Bay, and one voyage to Fiordland. They calculated the density, isotope ratios and biomass for more than 20 fish species including blue cod, tarakihi, red cod, trumpeter, leatherjacket, wrasses, sea perch and moki, as well as kelps and phytoplankton at the base of the food webs.

The team found that kelp forests provided the main source of organic matter in Fiordland fish communities with a majority of their biomass coming from kelp. In the Marlborough Sounds only a small amount of the organic matter came from kelp, with the rest from phytoplankton.

The researchers propose that one of the primary goals of ecosystem-based management (EBM) should be to maintain ecosystem connections between coastal reef fishes and kelp forest habitats.

Kelp in Fiordland are thriving in a relatively pristine environment with intact native forest catchments, clear water and a management plan providing a network of refuges for high trophic level fish and crayfish. In contrast, kelp forests in the Marlborough Sounds are declining, particularly over the last 50 years. They are being affected by stressors such as increasing sedimentation, marine heatwaves, and region-wide overgrazing by kīna (sea urchins). These stressors are related to human activities from land use, climate change and removal of kīna predators.

“Loss of kelp forest habitats affects the productivity of coastal fishes, We manage coastal fisheries based on the expectation of constant yields of fish populations, but our research demonstrates that the base of the food web is changing in some regions due to an increase in diffuse stressors,” says Project Leader Professor Stephen Wing.

“This means that the numbers of fish, productivity of those populations, and the variety of species in an area may change. EBM provides a framework to take these changes in ecosystem connectivity into account and adjust our expectations to what the system can provide under the stresses of life in a changing ocean.”

Understanding food web structure is vital for both management of biodiversity and resource use. These ecosystems are coming under increasing pressure from stressors. “Proactive measures are required now to build resilience before these systems pass a point of no return,” continues Steve continues.

The findings have been published in two open access articles, with a third coming out soon. A series of papers from a related study will take a more broad scale historical approach to understanding ecosystem connectivity in New Zealand’s EEZ, analysing fish specimens from museums and historic catch and survey data to investigate how the trophic structure of New Zealand fish commmunities have changed since the 1930s. Further research by this team is focusing on the consequences of expanding aquaculture production for coastal ecosystems in the Sounds.

 

Journal articles

Udy JA, SR Wing, SA O’Connell-Milne, LM Durante, RM McMullin, S Kolodzey & RD Frew (2019) Regional differences in supply of organic matter from kelp forests drive trophodynamics of temperate reef fish. Marine Ecology Progress Series 621: 19-32 DOI: 10.3354/meps12974

Udy JA, SR Wing, RM McMullin, SA O’Connell-Milne, S Kolodzey, LM Durante & RD Frew (2019) Organic matter derived from kelp supports a large proportion of biomass in temperate rocky reef fish communities: implications for ecosystem-based management. Aquatic Conservation: Marine and Freshwater Ecosystems 2019: 1-17 DOI: 10.1002/aqc.3101