2007, Marine Ecological Surveys Limited, Predictive Framework for Assessment of Recoverability of Marine Benthic Communities Following Cessation of Aggregate Dredging

Description

Over the last few years significant advances have been made in our knowledge of the environmental impacts of marine aggregate extraction. We now have a good understanding of the effect that dredging has on benthic communities within actively dredged areas. Our understanding of the recoverability of these communities is also advancing and whilst recovery to en exact replica of the pre-dredge community is unlikely, a similar community could be expected to have established itself within a number of years following the cessation of dredging activities. The structure of the benthic community which colonises a previously impacted area will be determined largely by the larvae that are available in the water column, as well as by species inhabiting neighbouring areas that are capable of colonisation through direct migration. In order to properly manage these marine resources it is important to understand the implications of the direct impacts of aggregate dredging on the benthos as well as the subsequent stages in marine food web recovery.  Very little is known of the role played by benthic taxa that characterise marine sands and gravels in supporting marine food webs. Information exists on the diet of only a limited number of fish species, mostly of commercial interest and rarely linked directly to benthic resources. An ecosystem approach has been employed here to investigate the ways in which marine fish are utilising benthic resources as food. The stomach contents of over 500 fish sampled with a 1m Beam trawl at aggregate extraction sites around the UK have been analysed. These were collected as part of two previous research projects funded through the ALSF (MEPF 04/02 & MAL0027) as well as through an environmental impact assessment carried out at Hastings Shingle Bank. These data were analysed in order to establish the diets of each of the 18 fish species studied in relation to the availability of prey. Diets were also compared between fish sampled in different environments in order to establish other sources of variation in their feeding behaviour. A strong link was established between the diets of individual fish and the available benthic prey. The diet of most of the fish species studied also showed a high degree of trophic adaptability, with a variety of prey items being utilised. Despite the variety evident in the prey consumption, crustacea were by far the most common prey type consumed. This result may have been exaggerated by the slow rate of digestion of this group compared to smaller soft bodied invertebrates. Nevertheless, with crustacea commonly accounting for 50% or more of the stomach contents, this group is clearly an important component of the benthos in terms of supporting higher trophic levels. In contrast to the dietary trends observed here, where crustacea are the dominant component, the benthos is typically dominated by polychaete worms, with crustacea being present only in quite small proportions. Crustacea, also tend to be larger in size and longer lived than polychaete worms meaning that they may take longer to recover. Therefore, whilst the opportunistic feeding behaviour observed in the majority of the fish indicates that they would cope well with alterations to the benthos the preference observed for crustacea may mean that this component of the benthos would need to recover before fish would use the area to feed. The role of the Annex I habitat Sabellaria spinulosa reef in supporting higher trophic levels was also investigated by comparing fish sampled in areas where reefs had been identified with those found in areas where reefs had not been identified. A significant alteration in the diet was observed in fish associated with S.spinulosa reefs which appears to be due to a slightly elevated level of feeding and an increased preference for the porcelain crab Pisidia. Pisidia has been shown to be associated with S.spinulosa reefs in very high abundances and it is therefore not surprising that it is targeted as a source of food in these areas. Whilst this study has advanced our understanding of the link between benthic communities and higher trophic levels, the number of observations was relatively small. Seasonal variability of fish diets is known to be high and this is something which has not been dealt with at all in this study as all fish were sampled during the summer months. It is possible that not only the diet, but also the feeding behaviour and trophic adaptability of fish may vary with the seasons and for this reason the results observed here should be treated with some caution. This study was also limited to smaller demersal fish which are easily caught with a scientific beam trawl; in order to understand the implications of aggregate extraction on the marine food web as a whole this would need to be extended to include larger pelagic fish species.