Recovery in the Population Abundance of Sensitive Fish Species

Life-history theory explains the evolution of species’ life-history traits under different mortality scenarios. Stable environments, with low disturbance mortality, support communities characterised by ‘K-type’ life-history traits (large body size, slow growth rate, late age and larger size at maturation, lower fecundity, etc.) while communities in regions of higher disturbance are more dominated by species with generally opposite ‘r-type’ traits (MacArthur and Wilson, 1967; May, 1976; Stearns, 1977, 1992; Roff, 1993; Huston, 1994; Reznick et al., 2002). Life-history theory predicts that heavily exploited fish communities (high disturbance mortality) should have fewer K-type and more r-type species. Elasmobranch species are generally characterised by K-type traits and many elasmobranch populations declined markedly during the 20th century (Frisk et al., 2001; Greenstreet and Hall, 1996; Walker and Hislop, 1998; Greenstreet et al., 1999a; Greenstreet and Rogers, 2000; van Strien et al., 2009). Teleost species with similar K-type life histories also declined (Philippart, 1998; Rijnsdorp et al., 1996). By the 1960s, average life-history trait composition among the demersal fish assemblage had become more r-type orientated (Jennings et al., 1999a; Greenstreet and Rogers, 2000, 2006; Greenstreet et al., 2012a), and in closely related pairs of species, the species with the more K-type life-history traits showed the greatest population decline at a time when fishing activity was high (Jennings et al., 1998). A species’ life-history trait composition provides an indication of its capacity to cope with additional mortality, and so determines its sensitivity to human activities that raise mortality rates above natural ambient levels. Species with K-type life-history traits are particularly sensitive to the additional mortality associated with fishing (Jennings et al., 1998; Gislason et al., 2008; Hobday et al., 2011; Le Quesne and Jennings, 2012). However, other pressures, such as climate change can also impact on populations of sensitive species (e.g., Bluemel et al., 2021).

Fishing activity is widespread across the assessment area and has been intense for a century or more (e.g. Rijnsdorp et al., 1996; Jennings et al., 1999b; Greenstreet et al., 1999b, 2009, 2011; Piet and Jennings, 2005; Shephard et al., 2011; Modica et al., 2014). As populations of fish species with K-type life-history traits across the region are likely to be in a depleted state, achieving acceptable status for these sensitive species will require population recovery. However, some species may be unable to sustain any level of fishing mortality, which means population recovery may not be possible for all sensitive species if any sort of sustainable fishing industry is to be maintained (Le Quesne and Jennings, 2012).

To support the assessment of the status of fish communities within OSPAR and additionally for the European Union Marine Strategy Framework Directive (MSFD), the workshop on the production of abundance estimates for sensitive species (WKABSENS) (ICES, 2021b) developed an assessment procedure based on a simple binomial test for temporal change in occurrence, derived from presence-absence data of species sampled by groundfish surveys operating across the geographic area to be assessed. A significant increase in occurrence of a sensitive species between assessment periods is considered an indication that the assessment thresholds have been achieved and the population is recovering in the area sampled by a survey. For species that are not considered unknown, a lack of any significant decline in occurrence between assessment periods can be considered commensurate to halting further population decline. An additional binomial test was then used to determine whether a population was recovering among a significant fraction of the surveys available within each OSPAR Region or, for deep-sea species, across the whole OSPAR Maritime Area.

Indicator metric and data collection

Approach for sensitive species

This assessment method (CEMP Guideline) has been updated since the IA2017 and is now calculated using records of occurrences (presences and absences) of species in catch data from standardized scientific groundfish surveys in order to capture changes in particularly rare species that were previously not assessed for data quality reasons. These species are commonly not well-sampled by the fishing gears employed (beam and otter trawls) due to low catchability and/or seasonal effect of survey timing. Therefore, the IA2017 methodology for this indicator and other assessment methods based on abundance estimates are not considered appropriate to detect temporal trends in data-poor species.

The 25 surveys used to provide data for the indicator, are long-term monitoring programmes that occur each year in the same period taking representative samples according to specific guidelines, but the temporal extent and spatial coverage of surveys vary (CEMP Guideline). No single survey covers the entire OSPAR Maritime Area and no single survey is best suited to catch all species. Therefore, multiple surveys are considered and assessed separately, and their assessment outcomes are integrated to provide a regional population status where appropriate. For surveys that were used in the IA2017, the assessment units have remained the same or are broadly similar.

A total of 102 unique taxonomic groups, divided by OSPAR Region, were identified as sensitive (ICES, 2021a,b; Rindorf et al., 2020; Greenstreet et al., 2012b) and included on the list to be addressed by this common indicator (CEMP Guideline). Species populations are assessed within Greater North Sea (OSPAR Region II) and Celtic Seas (Region III), as was done for the IA2017, with the addition of Bay of Biscay and the Iberian Coast (Region IV) and Wider Atlantic (Region V), where data are available. Although deep-sea species should ideally be assessed across all surveys and regions combined, deep-sea species assessments are made for the Porcupine Bank in the Wider Atlantic Region only, since the survey WASpaOT3 is the only one that consistently samples deep-water (> 500 m). For species where International Council for the Exploration of the Sea (ICES) or International Commission for the Conservation of Atlantic Tunas (ICCAT) quantitative assessments are conducted in all, or part of their distribution these assessments are considered more robust for identifying temporal trends in population abundance and, therefore, populations are not reassessed by this common indicator.

The change in the probability of occurrence is assessed through a binomial test (CEMP Guideline). Long-term and short-term assessments of change are made contrasting the mean occurrence in the last six years of the available survey data i.e., the “assessment period” against previous “reference periods” (CEMP Guideline). For the short-term assessment the reference period is created from the six years previous to the assessment period and for the long-term reference period all years before the assessment period are used. This approach was introduced by ICES Working Group on Abundance of Sensitive Species (WKABSENS; ICES, 2021b).

Thresholds (Recovering and No Further Decline)

By virtue of their sensitivity to additional human-related mortality, the population abundance and occurrences of each sensitive species sampled by each survey is assumed to have declined as a result of past human activities. There is good evidence that fishing mortality has indeed caused declines in the populations of sensitive species. Thus, long-term assessments related to population recovery constituted the primary basis for the assessment for a species within a survey area.

The assessment thresholds are the same as those considered for the IA2017. However, the thresholds per species are now assessed based on statistically significant change in occurrence rather than a percentile of abundance.

The primary threshold of the assessments is that sensitive species should be recovering over the long term (i.e., the threshold is achieved when demonstrating a statistically significant increase in occurrence).

A secondary assessment is also performed to address an alternative question of whether further decline in the occurrence of sensitive species has been halted (i.e., the threshold is achieved when a species is either stable (no significant increase or decrease) or recovering in the long term and must not be unknown).

Each species is assessed separately for each survey and both the primary and the secondary threshold is considered when data allows. Species are classified as “not assessed” if no data exists in the available surveys but the species is known to be present in the area. If fewer than five records are reported then the assessment outcome is classified as “unknown” (CEMP Guideline).

Each survey assessed has a differing start year, meaning that assessments of long-term change are not necessarily directly comparable between surveys or regions (CEMP Guideline). A temporally coherent measure of change that is comparable across the whole of the OSPAR Maritime Area is captured by the assessment of short-term change, which can highlight where signs of recovery or ongoing depletion are evident.

Spatial integration within species

The results from multiple surveys per species were integrated across each OSPAR Region, which constitutes the assessment units for the indicator, to identify the final result per species per Region. Deep-sea species assessments are made for the Porcupine Bank in the Wider Atlantic only based on one survey, therefore, no integration is undertaken for these species. To integrate outcomes a probabilistic “binomial integration” method is utilised, in which each survey is considered an equal indicator of change in the species population (CEMP Guideline).

An additional outcome, “mixed” was defined for the few instances of regional integration of surveys where there was significant evidence for both increases and decreases. This outcome was considered neither evidence for a “recovering” population or a “declining” population. Similarly, “mixed” differs from “stable” since stable results from neither a significant increase or decrease overall as opposed to evidence for both outcomes. When the “mixed” outcome from the integration is considered against the assessment threshold (see section: Thresholds (Recovering and No Further Decline), it results in “not achieved” for both “recovering” and “no longer declining”.