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The viability of fish populations relies on healthy habitats where juveniles can spawn, feed and develop. A cornerstone of sustainable fisheries management, these habitats are facing increased pressure from coastal development, climate change, pollution and a number of other factors.

It means there is an urgent need for better tools to understand which habitats really matter for both juvenile and adult fish populations so they can be conserved and managed. However, there are a number of challenges around how the quality of juvenile habitats is measured.

To try and address this, members of the International Council for the Exploration of the Sea (ICES) Working Group on the Value of Coastal Habitats for Exploited Species (WGVHES)—including

Dr. Benjamin Ciotti from the University of Plymouth—undertook a comprehensive review to evaluate the approaches being used to assess juvenile habitat quality.

They reviewed almost 900 studies from the past five decades covering juvenile fishes, crustaceans, and mollusks in marine and estuarine systems, with the goal being to evaluate how current science assesses habitat quality.

The resulting study, , highlights that despite widespread recognition of the ecological importance of juvenile habitats, the majority of existing studies employed relatively few metrics.

Around 85% of the research focused on measuring juvenile abundance, typically using methods like net surveys or diver-based visual counts. While this provides some information on whether a habitat is being used by a fish species, it does necessarily mean they are thriving in these areas.

The working group has called for more research factoring in the growth and particularly survival of fish, as while approximately half of the studies incorporated growth indicators, only 16% included measurements of juvenile survival. Crucially, fewer than 10% of the studies attempted to directly assess how juvenile habitats contribute to adult populations, which is widely considered to be the best metric of juvenile habitat quality.

This narrow focus, the scientists say, highlights a major gap in the evidence needed to evaluate habitat quality which is in turn leading to a mismatch between policy needs and available science, with management decisions often relying on incomplete or indirect indicators.

"The quality of juvenile habitats for fish populations can be measured in different ways, but none of them are easy. The fact that almost 10% of studies have attempted the best and perhaps most challenging metric—contribution to the adult population—shows that steps are being taken in the right direction. But we need much more information about the role that coastal habitats play in supporting young fish if we are to produce the robust information needed to put conservation in the service of building sustainable fisheries," says Ciotti.

This work sits at the heart of WGVHES's mission to support evidence-based protection of coastal habitats that underpin exploited species, and outlines a clear set of priorities for advancing juvenile habitat science.

It calls for moves beyond simple measures of abundance to include metrics like survival, growth, and contribution rates, as these factors more accurately reflect habitat quality. It also emphasizes the need to broaden spatial and temporal coverage of studies to capture shifts in habitat use occurring on scales ranging from seasons to tidal cycles.

To better assess habitat value, the authors advocate combining multiple indicators rather than relying on single metrics, and they highlight the potential of emerging technologies, such as tagging, telemetry, and stable isotope analysis, to fill critical data gaps.

Finally, the review underscores the importance of tracking developmental shifts during a fish's lifetime, recognizing that juvenile organisms often change habitats as they grow.

Taken together, the study's authors say it lays the groundwork for a more robust and biologically grounded approach to identifying essential fish habitats, and call for a new generation of monitoring and research to protect the habitats that matter most.

"We live in an age when new technologies such as molecular techniques, cameras and electronic tags are poised to offer tremendous insights into how juvenile fish use inshore habitats. But one method in isolation won't be the magical cure, and won't provide all the answers we need. It is the insights we can get from combining different approaches that will help us understand the process that support recruitment and the role that coastal habitats play in supporting this process," says Ciotti.