EU request on identifying operational indicators and defining usable threshold values (reference points) for criterion D3C3 under MSFD Decision (EU) 2017/848

ICES advises that fish populations with a “healthy population structure” under D3C3, as defined by EU (2017) and elaborated by EC (2022), are operationally characterized by a high proportion of relatively old/large individuals, wide age structures, high productivity, and the ability to resist and quickly recover from disturbances.

ICES advises that eight state indicators (retained after screening and testing) describing recruitment, growth of individuals, and age or length structure, are suitable to track changes in population structure in a wide range of populations. Depending on the population, one or more of these eight indicators may be necessary to capture all aspects of D3C3, with the selection of indicators dependent on population characteristics. Some of the retained indicators may not be suitable for populations with very short lifespans for which only two or three adult age classes would dominate spawning stock biomass (SSB) in the absence of exploitation, including populations of semelparous or near-semelparous species in which most individuals die immediately after reproduction. The retained indicators may be suitable for populations of elasmobranch species, which often have long lifespans, but the indicators were not tested for such populations. ICES notes that the list of retained indicators for addressing D3C3 is not exclusive because other potential state indicators exist and were not tested due to limited data availability.

Analysis of historical trends in six of the eight state indicators for 81 populations, simulation testing of five of the eight state indicators for seven of the 81 populations, and simulation testing of two of the eight state indicators for three additional Mediterranean populations demonstrated that values of the indicators covering recruitment, growth of individuals, and age or length structure were related to changes in fishing mortality (F; changes in F were based on a fixed exploitation pattern in both rounds of simulation testing). The lag-responses of age structure indicators to changes in F, which are typically slower than those of SSB, are inherent in the demographic properties of the populations that the indicators capture. The lag response is an important feature of age structure indicators for D3C3 because it will show when rates of F that meet targets for D3C1 have not been sustained for long enough to allow development of age structures consistent with meeting targets for D3C1 over the long term. While interpretation and additional data may be needed to understand the influence of environmental factors, ICES advises that the retained and tested indicators generally provide reliable information on one or more aspects of change in population structure in response to F. ICES also advises that the interpretation of trends in the retained indicators for age and length structure may benefit from information on trends in recruitment.

ICES is unable to advise on threshold values for any of the retained and tested state indicators because, based on the analyses performed, there are no clear change points representing a transition from a “healthy” to “unhealthy” population structure (e.g. critical transitions that lead to a relatively rapid change in state for a relatively small increase in F).

Since scientifically based threshold values cannot currently be defined, ICES is unable to determine when management in the context of CFP objectives is insufficient to achieve “healthy population structure” and therefore to advise on additional management actions (in addition to those for D3C1 and D3C2) to achieve “healthy population structure” (D3C3). However, the eight retained and tested indicators are suitable for describing aspects of change in population structures associated with fishing at or below FMSY, and the retained indicators for age and length structure are suitable for assessing whether a population has an age or length structure in accordance with meeting targets for both D3C1 and D3C2 over the long term.

Based on available evidence, ICES advises that the retained state indicators are suitable for detecting changes in productivity and the age and size structures of populations. Evaluation of the retained indicators primarily requires age structured stock assessment data, and this would currently limit application to many populations and regions, although data-limited approaches could be developed with further work.

Based on available evidence, ICES is unable to test and advise on a framework for comprehensively assessing D3 criteria for commercially exploited fish and shellfish populations. If thresholds are eventually established for some, or all, of the retained and tested indicators for D3C3, then a framework for assessing D3 criteria could be developed for populations with age-structured stock assessment data. Individual assessments could still be progressed at population and regional levels in the absence of a wider framework.