Preliminary assessment of discards survival in a multi-species Mediterranean bottom trawl fishery

Abstract

Summary The EU Common Fisheries Policy (CFP) reform establishes a gradual landing obligation for stocks for which catch limits are set, and in the Mediterranean for species with Minimum Conservation Reference Size. However, the landing obligation will not apply for species with scientific evidence of high survival rates. Discards estimates usually assume that the whole discarded catch is dead or dies at sea. However, survival of individuals discarded at sea is substantial for several species. In the Mediterranean Sea, the information on this issue is very limited. The aim of this study is to perform a preliminary assessment of the short‐term survival of discards in the multi‐species bottom trawl fishery in the Eastern Ionian Sea, Greece. Experimental fishing was held in spring when no water column stratification was observed and in autumn when a seasonal thermocline was formed. The catch was sorted, the regularly discarded species/sizes were put in tanks and their survival was monitored in specific time intervals up to 72 hr. A seasonal effect was obvious with high mortality in autumn being attributed to high water temperatures. Survival was low or zero for most species, but was relatively high for certain species (e.g., annular seabream, Diplodus annularis (Linnaeus, 1758); European conger, Conger conger (Linnaeus, 1758); brown comber, Serranus hepatus (Linnaeus, 1758) among bony fishes) and broad taxonomic categories (elasmobranchs and most invertebrates) in spring or in both seasons. Poor survival was generally estimated for most species subject to the landing obligation (e.g., hake, Merluccius merluccius (Linnaeus, 1758)). The present study, even though preliminary, can be important for improving future works, for identifying species with high survival potential, and—to a lesser extent—for the implementation of the CFP.

Escape, discard, and landing probability in multispecies Mediterranean bottom-trawl fishery

“…However, they suffer all the collateral effects from fisheries and, therefore, need to be subject to sustainable exploitation. It should be noted here that the majority of these species show low or negligible survival rates when discarded (Tsagarakis et al, 2018). The species examined in this work are not of high economic importance in the Mediterranean fishery, as they represent a low percentage of the total annual catches in the area (2.0%; FAO Fisheries Statistics 2017) and their commercial price is generally low.…”

Towards more selective Mediterranean trawl fisheries: are juveniles and trash excluder devices effective tools for reducing undersized catches?

“…However, although some studies have explored the survival rate of discarded species in Mediterranean trawl fisheries (e.g. Revill 2012, Tsagarakis et al 2018, Özvarol and Bolat 2017, there is little knowledge on the mortality rates of DPS and HKE that escape through sorting grids. This aspect should be closely investigated through experimental surveys in the near future to obtain an overall assessment of the effects produced by the adoption of the grids on the stocks of the two species (Vitale et al 2018).…”

An overview of bottom trawl selectivity in the Mediterranean Sea

“…Survival probability depended on species, fish size and water temperature, and was higher in red mullet (Metin et al, 2004) than in flat fish (Düzbastilar et al, 2016). A short-term survival assessment of different discarded species by Tsagarakis et al (2018), who monitored them in water tanks after sorting on board, mortality differed among species and showed strong seasonal variation, since higher water and air temperature severely affected survival.…”