Effects of chronic bottom trawling on benthic macrofaunal communities

Marine sediments constitute one of the largest ecosystems on earth. The animal life which inhabit these environments are known as benthic fauna, and represent a remarkable range of taxonomic and functional diversity. Bottom trawling is a fishing method where nets or collection devices are actively dragged over the seafloor to capture commercial fish or invertebrate species.

Commercial bottom trawl fisheries account for roughly one quarter of global seafood landings, and results in the widespread and chronic disturbance of seafloor habitats. In recent years, growing societal concern of the wider effects of fisheries has prompted the adoption of an ‘Ecosystem Approach to Fisheries Management’ (EAFM).

This has seen increased scientific interest and resources dedicated to the monitoring and assessment of fishery impacts to the wider ecosystem. Trawling impacts can, however, vary considerably across habitat type, fishing method, and environmental setting. Accordingly, an EAFM requires assessment methods that can ‘disentangle’ trawling effects from other natural and human drivers, and respond effectively to shifts in ecological quality.

The aim of the research outlined in this thesis was to examine which characteristics of benthic communities are most sensitive to bottom trawling. This was done using two intensive trawl fisheries in Denmark as casestudies; shellfish dredging in shallow coastal waters, and otter trawling for Norway lobster in the Kattegat.

Detailed benthic sampling programmes were undertaken to survey and analyse the response of benthic communities along spatially accurate gradients of trawling intensity. The results are presented in three research papers, which form the basis this thesis. In Danish nearshore waters, dredging for blue mussel and oysters constitutes the main bottom trawl fishery.

However, the fishing grounds here overlap with several marine protected areas, which are characterised by shallow, estuarine, and highly eutrophic conditions. Using high resolution fishing effort data, we investigated the relative effects of chronic dredging on benthic macrofauna at local and regional scales.

Dredging resulted in a significant negative effect on community biomass, and led to small but significant changes in community composition. However, we were unable to detect an effect of dredging to several community metrics, which may be explained by the widespread occurrence of several stress-tolerant and opportunistic taxa.

Furthermore, the observed trends were shown to vary considerably at local scales, demonstrating the importance of small scale assessments in nearshore areas. Benthic indicators are widely used in fisheries management to track and monitor the ecological effects of trawling. However, many widely used ecological indicators have been shown to be ineffective in this regard.

Given that trawling impacts to benthos are typically size-dependent, we size-separated benthic samples into ii small and large size fractions to investigate their respective sensitivity. We observed that indicators derived from large macrofauna were highly responsive to trawling disturbance, and were less influenced by other environmental drivers.

By contrast, indicators based on small individuals performed poorly, and those based on the whole community demonstrated a varied ability to detect trawling. The results highlight an underlying issue with using indicators of trawling disturbance based on the whole benthic community, and show how large benthic macrofauna can be used to improve indicator performance.

Biological traits are being increasingly used to interpret how changes in species composition can impact the functionality of ecosystems. Despite this, there are few examples of trait-based indicators being used to monitor and manage trawling impacts. Using size-separated benthic macrofauna, we examined the sensitivity of individual benthic traits to bottom trawling.

Benthic fauna which possess traits relating suspension feeding, tube-dwelling, a lack of mobility (sessile), and long-lifespans (>10 years) were particularly sensitive to trawling disturbance. We additionally validated our observations using an independent benthic monitoring dataset, and observed largely corresponding results.

This would suggest that chronic bottom trawling has a strong negative effect on specific traits of benthic macrofauna, and that data collected from a carefully designed one-off sampling event can provide results which are representative of long-term datasets. Given the importance of several of the above traits in processes such as benthic-pelagic coupling and sediment stabilisation, it is likely that loss of decline of these traits can have considerable implications for ecosystem functioning.

The thesis demonstrates that chronic bottom trawling has a significant impact on the structure, composition, and functioning of benthic communities. In addition, we show that the detection of fishery effects is challenging, and can be obscured by other manmade and environmental pressures. By focussing on the characteristics of benthos which are most sensitive to trawling, we present a number of potentially reliable trawling indicators.

The findings of this thesis are highly relevant to the management of trawl fisheries in Danish waters, and have the potential to improve methods currently used to assess and monitor fishery impacts and seafloor status at a wider scale.