Elsevier

Marine Pollution Bulletin

Volume 56, Issue 10, October 2008, Pages 1728-1736
Marine Pollution Bulletin

Impact of discards of beam trawl fishing on the nematode community from the Tagus estuary (Portugal)

https://doi.org/10.1016/j.marpolbul.2008.07.003Get rights and content

Abstract

The impact of dead discards, originating from beam trawl fishing on the nematode community from the Tagus estuary was investigated in terms of vertical distribution of the dominant nematode groups. Sediment cores were collected from a mud-flat from the Tagus estuary. Crangon crangon (Linnaeus, 1758) carcasses were added to the surface of the cores, simulating the settling of dead discards on the sediment. The vertical distribution of the dominant nematode groups was determined up to 4 cm deep at four different moments in time post deposition (0, 2, 4 and 6 h) and compared to control cores. The C. crangon addition to the sediment led to the formation of black spots and therefore oxygen depleted areas at the sediment surface. The Chromadora/Ptycholaimellus group, normally dominant at the surface layer, migrated downwards due to their high sensibility to toxic conditions. Sabatieria presented the opposite trend and became the dominant group at the surface layer. Since Sabatieria is tolerant to oxygen stressed conditions and high sulphide concentrations, we suggest that it migrated opportunistically towards an unoccupied niche. Daptonema, Metachromadora and Terschellingia did not show any vertical migration, reflecting their tolerance to anoxic and high sulphidic conditions. Our study showed that an accumulation of dead discards at the sediment surface might therefore alter the nematode community vertical distribution. This effect is apparently closely related to toxic conditions in the sediment, induced by the deposition of C. crangon at the sediment surface. These alterations might be temporal and reflect an adaptation of the nematode community to dynamic intertidal environments.

Introduction

Fisheries are an important provider of food, employment and income. An increasing number of studies are focusing on the side effects of fishing activities on the marine ecosystem (Kaiser et al., 2000, Jennings et al., 2001). One of these side effects is the generation of “bycatch”, which are accidentally caught species of no commercial interest (Hall, 1996). This bycatch is often returned to the sea as discards (Jennings et al., 2001).

The Tagus estuary is the only Portuguese estuary, where beam trawl fishing is allowed and the beam trawl is the most used fishing gear in the upper part of the estuary. Information about beam trawl fishing in the Tagus estuary is scarce and no estimates on catches and fishing effort (Cabral et al., 2002) are available. Total amount of fishery discards in the upper part of the Tagus estuary is estimated to be approximately 1500 ton per year, representing ca. 90% of the captures. This corresponds to an input of particulate organic matter of more than 140 ton of carbon and 35 ton of nitrogen per year (Cabral et al., 2002). Beam trawl fishing in the Tagus estuary is mainly targeted towards the soles Solea solea (Linnaeus, 1758) and Solea senegalensis Kaup, 1858 and discards are dominated by C. crangon (Linneaus 1758) (Cabral et al., 2002). After sorting on board, the bycatch is discarded. Mortality of C. crangon can be high and reach up to 96% (Gamito and Cabral, 2003). A large portion of the discard consists of carcasses which can attract different necrophageous species such as demersal fish and benthic invertebrates (Nickell and Moore, 1992, Kaiser and Spencer, 1996, Kaiser and Ramsay, 1997, Gamito and Cabral, 2003), pelagic fishes and dolphins (Hill and Wassenberg, 1990) or marine birds (Blaber et al., 1995, Garthe et al., 1996, Oro and Ruiz, 1997). An input of organic matter or general eutrophication can have a strong effect on the benthic community, altering diversity, density and community composition (e.g. Beukema, 1991, Palacín et al., 1992, Beukema and Cadée, 1997, Mazzola et al., 2000, Vanaverbeke et al., 2004).

Decomposition of C. crangon carcasses deposited on the sediment can create anoxic patches in the sediment (pers. observation), probably as a result of the oxygen consumption of the microbes responsible for the decomposition of these organisms. It has previously been reported that fine grained sediments easily become anoxic after the settling of organic matter (OM) at the sediment surface (e.g. Ólafsson, 1992, Van Duyl et al., 1992, Bickford, 1996, Kristensen, 2000, Steyaert, 2003).

Since oxygen plays an important role in the vertical distribution of intertidal nematode communities (Steyaert et al., 2005), we investigated the effect of the addition of C. crangon carcasses on the surface of Tagus sediments. Nematodes are known to migrate both horizontally and vertically. Horizontal migration was observed in recolonisation experiments (Schratzberger et al., 2000, Schratzberger et al., 2004), while vertical migrations were described as a response to tides (Steyaert et al., 2001) or changing oxygen concentrations (Steyaert et al., 2005). Here, we test the hypothesis that discards of C. crangon (through the triggering of anoxic patches in the sediment during the decomposition process) will not affect the vertical distribution of intertidal nematode communities.

Section snippets

Study site

Sampling was conducted in an intertidal mudflat of the Tagus estuary (38°44′N, 9°08′W) near Alcochete (Fig. 1) in March 2005 during ebb tide. The sediment temperature was 16 °C and the salinity of the interstitial water was 28‰. The surface of the sediment was sampled for pigment analysis, grain size, Total Organic Matter (TOM) and water content. For each analysis triplicates of 5 ml of sediment were collected. These samples were stored in the cold in the field and frozen (−80 °C) in the

Sediment characteristics and visual observations

The sediment had a medium grain size of 4.79 μm (SE = 0.02) with a water content of 63.4% (SE = 0.09) and a TOM content of 9.23% (SE = 0.12). The chlorophyll a concentration at the sediment surface was 29.2 μg g−1 (SE = 1.33).

In the bacterial experiment, after two tidal cycles, we could only observe the shrimps’ exoskeletons attached to the wires indicating that probably during that period the shrimps’ carcasses were eaten by other scavenger organisms leaving only the exoskeletons behind.

In the nematode

Visual observations and bacterial densities

When removing the shrimps from the sediment surface, black sediment spots were observed at the sediment surface, indicating reduced sediment. In a microcosm experiment with decaying macrofauna, Ólafsson (1992) also observed black spots when removing the dead macrofauna from the sediment surface. The colour of the sediment gives an indication of the redox state in the sediment, with the darker colours reflecting a more negative redox potential (Rosenberg et al., 2001, Diaz and Trefry, 2006).

Acknowledgements

We would like to thank the members of the Institute of Oceanography from the University of Lisbon for their support. We acknowledge R. Gamito and A. Rego for the help in processing the bacterial samples. UGENT-BOF project 01GZ0705 Biodiversity and Biogeography of the Sea (BBSea) (2005–2010). M.A. Franco was financially supported by a PhD grant from the Foundation for Science and Technology (FCT – SFRH/BD/10386/2002). The authors acknowledge the support by the MarBEF Network of Excellence

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