Unsupported Browser Detected

Internet Explorer lacks support for the features of this website. For the best experience, please use a modern browser such as Chrome, Firefox, or Edge.

What Fish Fat Can Tell About the Value of Deep-Sea Coral and Sponge Habitat

September 21, 2021

New research is advancing our understanding of how habitat influences productivity of commercially important rockfish in Alaska.

Photo of two researchers in waterproof coats and gloves using handheld instruments on a fish sample. NOAA Fisheries scientists assess body condition of rockfish using two different fish condition analyzers during an Alaska Fisheries Science Center bottom trawl survey. Credit: NOAA Fisheries.

Rockfish have an affinity for structure, whether it is created by rocks, corals, or sponges. But do deep-sea coral and sponge ecosystems offer benefits beyond structure? Are fish in these habitats more productive?

A new NOAA Fisheries study is the first to look at the relationship between fish condition and reproductive success in a variety of habitats, focusing on deep-sea coral and sponge ecosystems. As part of the study, scientists are developing methods to accurately assess rockfish condition by measuring fat content. For rockfish in Alaska, fat means healthy.

The study looked at the most commercially important rockfish in the Gulf of Alaska and Aleutian Islands: northern rockfish and Pacific ocean perch. Samples are being collected during 2021–2022 Alaska Fisheries Science Center bottom trawl surveys

The research addresses a priority of the North Pacific Fishery Management Council: understanding the importance of deep-sea coral and sponge ecosystems to commercially valuable fish. 

“Developing a feasible method to accurately measure fish condition across  Alaska waters during annual surveys will provide a wealth of data to help us understand how habitat influences fish productivity. That knowledge will also help us track how climate change is affecting the ecosystem,” said study leader Christina Conrath, NOAA Fisheries biologist at the Alaska Fisheries Science Center. 

Underwater photo of a rockfish peering out from behind coral attached to a rock.
A northern rockfish shelters under rock and coral. Credit: NOAA Fisheries

Exploring the Influence of Coral and Sponge Habitat on Fish Productivity

The new project is part of NOAA’s Alaska Deep-Sea Coral and Sponge Initiative, which is supported by the Deep Sea Coral Research and Technology Program. The initiative was established to provide the scientific data needed to inform management and protection of deep-sea coral and sponge ecosystems in Alaska. Discoveries made previously and under this initiative have greatly advanced our knowledge of corals and sponges and their role in Alaska marine ecosystems. 

Rockfish in Alaska waters are frequently found in coral and sponge habitat. Previous Alaska Fisheries Science Center research found that rockfish densities were highest in structurally complex habitat. But that research showed no evidence that structure created by corals and sponges was more important than that formed by rocks. 

“We know that rockfish get value from structure. But we don’t really have evidence yet that coral and sponge habitat offers benefits beyond structure,” Conrath said. “We’re exploring that.”

Underwater photo of a pink colored Pacific ocean perch above a sandy sea floor.
Pacific ocean perch. Credit: NOAA Fisheries.

Linking Rockfish Condition and Reproduction

For Alaska rockfish, fat indicates healthy body condition. Fish condition is thought to be a predictor of productivity. Reproduction costs a lot of energy. Evidence suggests that fish with higher energy reserves—fat—are more reproductively successful, and therefore more productive. 

Several rockfish species in Alaska waters have been shown to skip spawning  in some years. Scientists don’t yet know if this reproductive failure is related to body condition. 

The new study will measure both body condition and reproduction to quantify the link between them for these rockfish species.  

Developing an Accurate Method to Assess Rockfish Condition

An accurate and practical way to assess fish condition across Alaska marine ecosystems would be an invaluable tool to advance understanding of essential fish habitat for commercially important Alaska fish species.


Research led by Alaska Fisheries Science Center biologist Jerry Hoff aims to develop such a tool for Pacific cod and Alaska pollock. The rockfish study builds on this research, using similar methods for northern rockfish and Pacific ocean perch. 

Simple ways to measure fish condition use weight, liver weight, and fish length. A better way to measure fish condition may be to estimate the amount of fat using a fish quality analyzer. The study is examining two different tools. The Yamato Fish Analyzer measures impedance (resistance to electrical currents) in fish flesh. The Distell Fatmeter measures water content. 

Photo of Yamato Fish Analyzer handheld device.
The Yamato Fish Analyzer. Credit: NOAA Fisheries.
Photo of the Distell Fatmeter handheld device.jpg
The Distell Fatmeter. Credit: NOAA Fisheries.

“We are using both measurements to estimate lipid (fat) content; that’s the number we are after. We are trying to find out if these measurements will be more accurate than simpler indices of condition.” Conrath said. “At Auke Bay Laboratories we will validate Fatmeter and Fish Analyzer results and other condition indices against direct measures of lipids. That will help us find what measurement gives us the best answer and is feasible on surveys.”

“Our findings will further our understanding of how important deep-sea coral and sponge habitat is. It will feed into efforts to move toward ecosystem based management. It will give us tools to understand how fish condition changes over time and space with climate change,” Conrath said. “If we can monitor fish condition, we can see what is happening with different species. We can see who is doing well and who is doing poorly. We can adapt and prepare for future change.”