Impact of discards of beam trawl fishing on the nematode community from the Tagus estuary (Portugal)
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
References (49)
The effects of sewage organic matter on biogeochemical processes within mid-shelf sediments offshore Sydney, Australia
Marine Pollution Bulletin
(1996)- et al.
N2, N2O and O2 profiles in a Tagus estuary salt marsh
Estuarine Coastal and Shelf Science
(1999) - et al.
Influence of bacterivorous nematodes on the decomposition of cordgrass
Journal of Experimental Marine Biology and Ecology
(2003) - et al.
Comparison of sediment profile image data with profiles of oxygen and Eh from sediment cores
Journal of Marine Systems
(2006) - et al.
Mortality of brown-shrimp discards from the beam trawl fishery in the Tagus estuary, Portugal
Fisheries Research
(2003) - et al.
Composition, distribution, biomass and production of the North Sea meiofauna
Netherlands Journal of Sea Research
(1990) - et al.
The behavioural ecology of epibenthic scavenging invertebrates in the Clyde Sea area: laboratory experiments on attractions to bait in static water
Journal of Experimental Marine Biology and Ecology
(1992) - et al.
Evidence for coincidence of meiofauna spatial heterogeneity with eutrophication processes in a shallow-water mediterranean bay
Estuarine Coastal and Shelf Science
(1992) - et al.
Response of benthic fauna and changing sediment redox profiles over a hypoxic gradient
Estuarine Coastal and Shelf Science
(2001) - et al.
The importance of fine-scale, vertical profiles in characterising nematode community structure
Estuarine Coastal and Shelf Science
(2003)
Responses of intertidal nematodes to short-term anoxic events
Journal of Experimental Marine Biology and Ecology
The impact of organic matter and macrozoobenthos on bacterial and oxygen variables in marine sediment boxcosms
Netherlands Journal of Sea Research
Changes in composition of bottom fauna of a tidalflat area during a period of eutrophication
Marine Biology
Local differences in macrozoobenthic response to enhanced food supply caused by mild eutrophication in a Wadden Sea area: food is only locally a limiting factor
Limnology and Oceanography
Trawl discards in the diets of tropical seabirds of the northern Great Barrier Reef, Australia
Marine Ecology-Progress Series
Importance of discards of a beam trawl fishery as input of organic matter into nursery areas within the Tagus estuary
Hydrobiologia
Top–down impact of bacterivorous nematodes on the bacterial community structure: a microcosm study
Environmental Microbiology
Trophic resource and position of metazoan meiobenthos at contrasting subtidal sediments: carbon and nitrogen stable isotope analysis
Marine Ecology-Progress Series
Amounts of discards by commercial fisheries and their significance as food for seabirds in the North Sea
Marine Ecology-Progress Series
On bycatches
Reviews in Fish Biology and Fisheries
The ecology of free-living nematodes
Oceanography and Marine Biology: An Annual Review
Vertical distribution of the nematode fauna in a coastal sediment influenced by seasonal hypoxia in the bottom water
Ophelia
Fates of discards from prawn trawlers in Torres Strait
Australian Journal of Marine and Freshwater Research
Cited by (15)
Effects of widespread human disturbances in the marine environment suggest a new agenda for meiofauna research is needed
2020, Science of the Total EnvironmentCitation Excerpt :For example, mixing of sediments following the passage of fishing gear releases previously buried organic material and benefits those species that are able to use this surplus either directly or via increased primary production. Similarly, increased organic loading of the sediment can lead to a shift towards microbial-dominated, anaerobic food chains, resulting in the proliferation of the few species that can survive in oxygen-poor sediments and are able to exploit microbial food sources (Franco et al., 2008). Results from a mesocosm experiment suggested that removal or reduced densities of larger macrofauna species as a result of bottom fishing may lead to increased nematode abundance, most likely mediated by increased accessibility to food in the absence of macrofauna (Ingels et al., 2014), although evidence for close relationships between meiofauna-sized and larger organisms in the field is equivocal (Austen et al., 2003).
An experimental protocol to select nematode species from an entire community using progressive sedimentary enrichment
2016, Ecological IndicatorsCitation Excerpt :Steyaert et al. (2005) also demonstrated that T. communis, which is another close species to T. longicaudata, is a typical inhabitant of poorly oxygenated sediment, which visibly supports its low oxygen requirements. In contrast, recent experiments have revealed that the Ptycholaimellus species either migrate away from anoxic spots in the sediment (Franco et al., 2008) or experience high mortality in experimental anoxic/suboxic set-ups (Steyaert et al., 2007). However, the oxybiotic behaviour of P. ponticus does not mean that it should be present in the superficial sediment layer, as Hope and Murphy (1969) clearly proved that this chromadorid has a system of body pores and hypodermal glands that can modify sedimentary microenvironments by building well-oxygenated tubes.
Structure and taxonomic composition of free-living nematode and macrofaunal assemblages in a eutrophic subtropical harbour, Hong Kong
2014, Marine Pollution BulletinCitation Excerpt :In association with organic enrichment, oxygen stress may also hinder the development of epigrowth feeding and predatory nematodes in the study area. Franco et al. (2008) found that epigrowth-feeding nematodes (genera Chromadora and Ptycholaimellus) can migrate away from anoxic sediments. Higher mortality of epigrowth-feeding nematodes has also been reported from experimental studies under anoxic/suboxic conditions (Steyaert et al., 2007).
Sediment-related distribution patterns of nematodes and macrofauna: Two sides of the benthic coin?
2011, Marine Environmental ResearchCitation Excerpt :Therefore the absence or low densities of epistrate feeding and predatory nematodes cannot be attributed to a lack of food particles and we suggest that the oxygen stressed environment hampers the development of nematode communities that include high densities of epistrate and predatory nematodes. This is supported by the observation that 2A nematodes (Chromadora/Ptycholaimellus) migrate away from anoxic spots in the sediment (Franco et al., 2008b) or experience high mortality in experimental anoxic/suboxic set ups (Steyaert et al., 2007). Deposit-feeding nematodes are not limited to fine-grained sediments (Table 3), however different species contribute importantly to within size class similarity in fine and coarse sediments, suggesting that deposit-feeding itself is not an adaptation to surviving oxygen stress.
Spatial and seasonal variations of subtidal free-living nematode assemblages in the northern Beibu Gulf, South China Sea
2012, Journal of the Marine Biological Association of the United KingdomHidden Players—Meiofauna Mediate Ecosystem Effects of Anthropogenic Disturbances in the Ocean
2023, New Horizons in Meiobenthos Research: Profiles, Patterns and Potentials