Indicators of stock status for large-pelagic fish based on length composition from driftnet fisheries in Zanzibar

Small-scale fisheries (SSF) contribute to approximately half of the total landings of tuna and tuna-like species in the Indian Ocean and are an important form of employment and source of protein. Research into the properties and dynamics of SSF in East Africa are important for the assessment and sustainable management of fish stocks, however, detailed fisheries data are often inadequate or absent. Fisheries-dependent data on driftnet fisheries in Zanzibar, Tanzania, was collected during the northeast monsoon seasons in 2014 and 2015. The data describes the properties of the driftnet fisheries and allows for comparisons of the length composition of the landings of the SSF with large-scale industrial fisheries (IF) fishing in Tanzania’s Exclusive Economic Zone (EEZ). This data also facil- itates the calculation of stock indicators for the five most abundant tuna and tuna-like species landed in Zanzibar. Results show that the two fisheries (SSF and IF) exploit the same stocks, and landings are representative of a similar length composition, while operating in different parts of Tanzania’s EEZ. High exploitation rates, above reference levels for all species were calculated, in agreement with official assessments by the IOTC, and suggest that calls for the expansion of the SSF should be reconsidered. The assessment and management of straddling stocks are dis- cussed, as well as solutions to challenges faced by local observer programmes

Testing and comparing biological seabed indicators in response to bottom trawling in the Jammer Bay

A range of biological indicators have been developed and applied to evaluate the condition of the seabed in Danish coastal waters. However, some of these indicators were primarily designed to measure their response to eutrophication, a stressor that is not always the most prevalent factor in areas where bottom trawling occurs. Recently, new indicators have been developed specifically to assess the impact of trawling on the benthic community. In this study, we aim to test and compare a variety of biological seabed indicators along a trawling gradient, while also taking into account natural conditions, seabed habitat types, and other stressors, such as nutrient and oxygen concentrations. The study is situated in Jammer Bay and uses high-resolution spatial fisheries data and benthic fauna sampled collected during three recent field campaigns in the area. The results are expected to enhance the national assessments of the seabed in Danish coastal waters

Using large benthic macrofauna to refine and improve ecological indicators of bottom trawling disturbance

Bottom trawling alters the abundance, diversity, size-composition, and function of benthic communities. However, the ability to detect these impacts over large spatial scales can be obscured by various complicating factors, such as community adaptation to disturbance and co-varying environmental conditions. An ecosystem-based approach to fisheries management therefore requires ecological indicators which can ‘disentangle’ trawling effects from other natural and human drivers, and respond effectively to shifts in ecological quality. We collected benthic macrofaunal samples at 21 sites across a Norway lobster Nephrops norvegicus fishing ground in the Kattegat, and separated the benthic community into small (1–4 mm) and large (>4 mm) size fractions. Four taxonomic indicators (total density, species density, Shannon diversity, and biomass) and four functional indicators (functional diversity, functional richness, functional evenness, and functional dispersion) were calculated based on each size fraction, and the two fractions combined (pooled community). Here, we compare the ability of these indicators to detect trawling impacts across size categories. We show that indicators derived from large macrofauna were highly effective in this regard, and were less influenced by other environmental drivers, such as depth, sediment grain size, bottom current velocity, salinity, and temperature. This suggests that the taxonomic and functional characteristics of benthic communities display a size-dependent sensitivity to trawling disturbance, and therefore community metrics based on large benthic macrofauna may provide useful indicators. By contrast, indicators derived from the small fraction performed poorly, and those based on the pooled community demonstrated a varied ability to detect trawling. Small macrofauna are typically characterised by high density, diversity, and population growth rates, and their relative resilience to trawling may mask the response of the more sensitive macrofauna. This highlights an underlying issue with calculating indicators based on the whole benthic community. The approach outline here is easily applied, improves indicator performance, and has the potential to reduce laboratory workloads due to the fewer taxa and individuals required for analyses