High bycatch rate of the coral Cladocora caespitosa offsets the low discards ratio in Thermaikos Gulf gillnet fishery

Abstract

While hanging nets may have fewer overall environmental impacts than towed gears, they still catch unwanted species and sizes, which are then discarded. Gillnets are one of the most common types of nets used in Mediterranean small-scale fisheries, with commercial catches and discards containing a diverse range of fish and invertebrate species. In this study, the catch profile of gillnets in the Thermaikos Gulf (NE Mediterranean) was analyzed with a focus on the discarding of species with unfavorable conservation status, such as the Mediterranean-endemic scleractinian coral Cladocora caespitosa, which is listed as endangered due to declining population sizes. Our survey was conducted over two seasons (May to October in 2020 and 2021) and included 69 fishing trials under realistic fishing conditions. In general, gillnets exhibited a relatively low discards ratio (17.2%; 7.1% when only fish were considered) and a low prevalence and capture intensity of endangered or threatened species; during the entire survey, only one chondrichthyan was captured (1 specimen of Raja radula), while only two out of 54 fish species were of vulnerable IUCN status. These advantages were offset in part by the high catchability of C. caespitosa colonies, which occurred in 61% of hauls and accounted for 53% of total invertebrate biomass and 30% of total discarded biomass. The likelihood of hauling living colonies of C. caespitosa was higher when fishing over known coral beds (81% vs. 43%), and the overall probability of hauling C. caespitosa in any state was 18% higher over coral beds, suggesting that gillnets frequently detach colonies from the seafloor, which are then dispersed over a larger area as fragments with probably low survival potential. Our findings highlight the importance of identifying C. caespitosa beds so that fishermen avoid setting their nets in these areas, provided that they are well-informed and educated about the ecological significance of this endemic, threatened, and ecologically important species.

Mitigating bycatch in Mediterranean trammel net fisheries using species-specific gear modifications

“…Our third experimental net, which was fitted with a guarding net between the netting panel and the footrope, was ineffective at reducing batoid captures. This contradicts our initial premise because a recent study conducted by our team [36] show that local batoids are not trapped by gillnets, leading us to believe that the guarding net, which is a short strap of gillnet, would prevent the capture of these species. However, it appears that instead of leaping off the webbing of the guarding net, batoids swim upwards to avoid this impediment, becoming caught in the trammel net.…”

“…The first modified gear (SELV; figure 2b) was a standard trammel net with a guarding net (gillnet, 40 mm polyethylene mesh, 2.5 mesh high) fitted between the net and the footrope. The theory for assessing this modification was that gillnets only rarely catch rays, at least in the surveyed area [36], and because the selvedge is known to create a barrier zone for benthic scavengers to reach royalsocietypublishing.org/journal/rsos R. Soc. Open Sci.…”

Evaluating the Sustainability of an Eastern Mediterranean Gillnet Fishery Based on the Catches of Undersized Individuals and the Reproductive Period of Targeted Species

“…Most of the non-targeted species were caught sporadically and in very low numbers. The discards ratio was low, and there was a low prevalence and capture intensity of endangered or threatened species [27].…”

“…Gillnets employed in SSFs, like any other fishing gear, are not selective for any given species or size range [31,32]. In the Thermaikos Gulf, gillnets: (i) fail to spare undesired species with no commercial value in the Greek market, but with a low discard ratio and prevalence/capture intensity of endangered or threatened species [27], (ii) they catch undersized individuals of species whose management is under regulation, and (iii) their operation period overlaps with the GSI peak of several exploited species. On the contrary, the surveyed métier catches its main target species, M. surmuletus, almost exclusively above the established MCRS and after the peak of its spawning activity, rendering its exploitation sustainable.…”

Oceanographic Research in the Thermaikos Gulf: A Review over Five Decades

“…along the gulf represent the hard substrata type of the seabed. The hard substrata are covered by periwinkles, limpets, and barnacles in the supralittoral and mediolittoral zones [150], whereas in the sublittoral zone, four different facies of the photophilic algae community develop, depending on prevailing environmental factors [151][152][153][154].…”