Trawling modifies red algae bed fauna and flora off Balearic Islands
2017-12-06 11:55:55   copyfrom:    hits:

Red algae obtained in the zone studied (Photo: IEO)SPAINMonday, November 13, 2017,23:00 (GMT + 9)A group of rese

Red algae obtained in the zone studied. (Photo: IEO)


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Monday, November 13, 2017, 23:00 (GMT + 9)


A group of researchers that carried out a multidisciplinary study on the impact of trawling on the continental shelf of southern Mallorca has shown that, despite the fact that red algal beds off the Balearic Islands persist in the traditional fishing grounds of the trawling fleet, this fishing exploitation has modified its flora and fauna.

The analysis of the fishing footprint, obtained from the monitoring system of the trawl fleet via satellite ("blue boxes"), has made it possible to locate two adjacent zones, similar in the geomorphology from the seabed and in their oceanographic characteristics, but with different fishing exploitation degree. While one is located in the so-called "Pesquera Rica", one of the traditional fishing grounds of the trawling fleet whose fishing exploitation dates back to the beginning of the 20th century, the other is an area not subject to exploitation with trawl gear because it is protected in a natural way by a series of rocky outcrops that delimit it.

Researchers from the Oceanographic Centre of the Balearic Islands and the Central Services of the Spanish Institute of Oceanography (IEO), in collaboration with the Institute of Marine Sciences (ICM) and the University of Girona, compared the surface sediments of the seabed and the benthic communities of flora and fauna in the impacted and non impacted areas. In addition, they analyzed the information collected by scientific observers on board the trawling fleet operating in the impacted area.

In both areas, in the red algae communities, Corallinales species and the Peyssonneliacea family, predominate, which are included in the so-called Peyssonnelia seabeds, known by fishermen as hazelnut and herba torta beds, widely distributed on the Balearic shelf between 50 and 80 metres deep, where they overlap with fishing grounds of the trawling fleet.

These algae groups account for more than 90 per cent of the total algae biomass, both in the impacted area and in the non impacted one by trawling. However, the impacted area showed a reduction in algae biomass of up to 40 per cent compared to the non-impacted area as well as a reduction in its diversity. This same trend was observed in the abundance and biomass of most benthic fauna groups, which decreased in the impacted area, but not in diversity.

Changes were also detected at the level of species composition. In general, the benthic species with greater presence in the trawling fleet catches are species that showed low abundance and biomass in the impacted area, where it seems that they have been replaced by smaller species, rarely captured by commercial fishing. This would be the case of the urchin Spatangus purpureusor the ascidian Polycarpa mamillaris, the most abundant benthic species in the non-impacted area, whose abundance in the area subject to fishing exploitation decreases drastically, while that of smaller species, such as small brittle stars and the ascidian Aplidium nordmanni, increases.

The reduction of soft red algae species belonging to the genus Peyssonnelia in the impacted area is explained by the fact that the algae are more captured by the trawling fleet in the study area.

On the other hand, the decrease of coral algae that is also observed in the impacted area, some of them protected by European environmental regulations, can not be explained in the same way since its presence in commercial catches is scarce. The resuspension of sediments, caused by the passage of the trawling gear on the seabed, and its subsequent deposition, could have caused the decrease of these algae, which are very sensitive to their coating, which causes loss of photosynthetic capacity and, therefore, it endangers their survival.

There are two reasons that explain the coexistence of red algae seabeds and trawling in the Balearic Islands. On the one hand, it could be because the effort of this fishery in the archipelago is relatively low compared to other areas of the western Mediterranean, such as the Iberian Peninsula. On the other hand, due to the fact that with the implemented routines and the fishing gear used, an attempt has been made to avoid excessive contact of the trawling net with the bed so as not to capture algae as much as possible, which hurdles and lengthen the onboard triad of the catches and decrease their quality.

However, the researchers note that "despite this coexistence between trawling and red algae beds in the Balearic Islands, the study shows that the fishery, even at a relatively low intensity, has a negative impact on these beds, which also seem to act as essential habitats for the sustainability of the marine resources themselves

Therefore, they reiterate the need to take urgent measures to preserve the red algae seabeds in the Balearic Islands and to make this protection compatible with the sustainability of trawling.

"It is necessary to map the benthic habitats and the fishing grounds in order to identify conflict areas in which to implement management plans that include not only spatial planning measures to diminish the direct interaction between the trawl fishery and these habitats, but also technical improvements in fishing gear, with the aim of reducing its impact on the seabed and benthic species without commercial interest", concludes Francesc Ordines, first author of the study.

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