Project
title / Framework for evaluating the application of seasonal or rotational scallop fishery closures
/ DEFRA
project code / MF0228

Department for Environment, Food and Rural Affairs CSG 15

Research and Development

Final Project Report

(Not to be used for LINK projects)

Two hard copies of this form should be returned to:
Research Policy and International Division, Final Reports Unit
DEFRA, Area 301
Cromwell House, Dean Stanley Street, London, SW1P 3JH.
An electronic version should be e-mailed to
Project title / Framework for evaluating the application of seasonal or rotational scallop fishery closures
DEFRA project code / MF0228
Contractor organisation and location / The Centre for Environment, Fisheries and Aquaculture Science
Lowestoft Laboratory, Pakefield Road
Lowestoft, Suffolk, NR33 0HT
Total DEFRA project costs / £ 30,020
Project start date / 01/01/02 / Project end date / 31/03/02
Executive summary (maximum 2 sides A4)
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CSG 15 (Rev. 6/02) 4

Project
title / Framework for evaluating the application of seasonal or rotational scallop fishery closures
/ DEFRA
project code / MF0228

(1) The purpose of the project is to provide a framework for the evaluation of the potential benefits of temporary, seasonal and permanent closures of parts of scallop fishing grounds. Potential benefits are considered in terms of: (i) enhanced fisheries productivity and protection of scallop spawning stocks; and (ii) amelioration of the impacts of scallop dredging on benthic environments and biota;

(2) The first part of the project focuses on the fishery outcomes of spatial management. The outcomes considered are firstly the abundance and population structure of the scallop stock, particularly the spawning component, and secondly the quantities and structure of fishery yield. Using an approach similar to that developed by Caddy & Seijo (1998), scientists from The Centre for Environment, Fisheries & Aquaculture Science (CEFAS) constructed a spatially structured simulation model describing the population dynamics of a scallop stock and its responses to fishing under a variety of spatial and other types of management scenarios. The aim of the model is to provide a framework for addressing questions about the effectiveness of spatial management strategies and about their interactions with other management measures.

(3) For ease of formulation, population dynamics in the model are described in terms of age rather than size. However, since scallop fishery selection patterns are related to size rather than age, size-based processes are linked to age through model inputs describing seasonal patterns of growth. Biological and fishery parameters for the model are drawn from published sources and survey and research data held by both Port Erin Marine Laboratory (PEML) and CEFAS. Parameter values were chosen to represent an Irish Sea-like scallop stock.

(4) Targeting of fishing effort in relation to the spatial distribution of the stock is modelled by a profit maximization algorithm. According to this algorithm, units of fishing effort (trips) are directed according to a trade-off between the potential landings per unit effort (LPUE) and the cost of fishing at a particular location.

(5) Both direct (landings) and indirect fishing mortality is considered in the model. Indirect fishing mortality is divided into discard and impact mortality. The latter is the mortality of scallops that are not seen in the catch yet are nevertheless impacted by fishing operations. Modelling of indirect mortality allows us to assess management effects on scallop ‘wastage’ rates. As well as mortality inflicted by the targeting scallop fleet, the model allows fishing mortality from a by-catch fleet – notionally targeting queens.

(6) Spatial variation in scallop density is maintained by distributing recruits around a pre-defined number of randomly located recruitment patch centres. Total numbers of recruits are fixed as a model input, since there is insufficient information on the stock-recruitment relationship.

(7) Management options implemented within the model include closed areas (rotational or permanent), closed seasons, effort limits, TACs, harvest limits and minimum catch rate limits. These are applied at the spatial level of ‘grounds’, each ground occupying one or more spatial units (grid squares) of the overall stock area.

(8) Simulations based closely on the main Irish Sea scallop fishery around the Isle of Man show that exploitation of a virgin stock quickly causes a shift towards younger individuals in the population and catch. The simulation results are similar to historical observations on an Irish Sea ground from the early days of exploitation in the 1930s. Simulated LPUEs are much lower than currently recorded in the Irish Sea. This appears to be due mainly to underestimation of average recruit densities rather than to errors in specification of fishery parameters.

(9) A simplified stock simulation, still based on Irish Sea parameters, was used to explore the effects of various spatial and temporal management scenarios – permanent and rotational closed areas and closed seasons. Rotation scenarios differed according to number of grounds, length of closure for individual grounds, proportion of the total stock area closed and rotation period.

(10) The principal benefit of spatial management (and closed areas) appears to be enhanced spawning stock biomass (SSB) rather than improved fishery yield. Increases in average yield from rotational closures come at the expense of decreased stability of yield between years.

(11) Both SSB and inter-annual variability of yield increase with length of closure and proportion of the total area closed. At the effort levels considered in the simulations, yield increases slightly at low closure proportions and short closure lengths.

(12) Permanent closures deliver increases in SSB more effectively than rotational closures. Permanent closures also do not increase inter-annual variability of yield, although (ignoring potential recruitment benefits) they may decrease the average yield. Concentration of fishing effort in areas outside the permanently closed areas results in increased wastage rates through dredge impacts and discarding of scallops.

(13) Further research is needed into the foraging patterns underlying the spatial targeting of fishing effort. In reality, fishing patterns are likely to be less optimal than assumed under this model, and this may have the effect of dampening the inter-annual variability of yield.

(14) Further research is also needed into scallop recruitment patterns: How stable is the distribution of recruitment between years? What kind of feedbacks exist between spawning stocks and recruitment? If settlement of juveniles is enhanced in unfished areas, what are the implications for recruitment to the fishery?

(15) The management scenarios examined in these simulations are not exhaustive. The model provides the framework for simulations of many other scenarios, which should include consideration of a range of levels of fishing effort.

(16) The second part of the project focuses on reducing the benthic and environmental impacts of scallop dredging. Extensive studies in the Irish Sea by researchers at PEML have demonstrated that scallop dredging alters benthic communities in both the short- and long-term. In this project PEML scientists have collated and evaluated information from these and other relevant studies, considering evidence on the likely time scales for recovery and the consequent implications for spatial management of scallop fishing. This has enabled future research needs to be identified and has shed light on the methodologies needed to monitor and evaluate the practical implementation of closed area management.

(17) Short-term rotational closures (< 4 years) can increase habitat complexity and the production and diversity of benthic species. Communities on sandy and gravelly substrates may show good recovery at this time scale and commercially important species like queens and edible crabs could also benefit. However, communities are likely to be dominated by robust, fecund or fast growing opportunists.

(18) Longer rotational closures (5-10 years) should benefit more fragile or longer-lived, less fecund organisms including scallops, whelks and anglerfish, together with complex communities with turf forming species like horn wrack or long-lived infauna like fan mussels.

(19) Permanent closure may be necessary to protect biogenic reefs and areas of bedrock where long-lived sessile species are common. Permanent closure should also benefit adjacent grounds, through export of adult scallops or larvae as their fertilization success, growth and recruitment improve within closed areas.

(20) Seasonal closures appropriate for scallop fisheries may benefit other organisms with similar life histories. Advantages for the whole benthic community are probably minimal because many species are vulnerable to dredging outside the seasons when scallops should be protected. However, any measure that has the effect of decreasing overall fishing effort may hold some benefits for benthic diversity and production.

(21) Research is needed to identify where target organisms (scallops, crabs, sea fans etc.) recruit from so that sources of recruitment rather than sinks are conserved.

(22) Where habitat preservation is a priority, noticeable keystone or sensitive species should be identified and used to monitor recovery.

(23) There is an urgent need for more information on the recovery rates of temperate marine reserves. Existing information is often limited to single locations and involves minimal replication (temporal or spatial) and inadequate statistical controls. Rotational closure is even less studied and there are no detailed descriptions of its effects for benthos other than the target organisms. Properly replicated experiments should be carried out to determine rates of recovery of commercially important species (e.g. scallops, edible crabs, anglerfish, whelks) and habitat indicators (e.g. maerl, upright bryozoans).

CSG 15 (Rev. 6/02) 4

Project
title / Framework for evaluating the application of seasonal or rotational scallop fishery closures
/ DEFRA
project code / MF0228
Scientific report (maximum 20 sides A4)
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CSG 15 (Rev. 6/02) 4

Project
title / Framework for evaluating the application of seasonal or rotational scallop fishery closures
/ DEFRA
project code / MF0228

Background

Spatial management of fishing effort is increasingly recognised as a valuable tool for controlling access to exploited marine resources (National Research Council, 2001). Such management includes temporary and permanent closures of parts of fishing grounds, recognised as particularly important for sedentary stocks such as benthic shellfish (Orensanz & Jamieson, 1998). A concomitant benefit of closures is that they may allow recovery of impacted environments and biological communities (Collie et al., 1997).

Rotational and other closures of scallop fishing grounds have already been demonstrated to benefit scallop stocks and fisheries elsewhere in the world. Short rotations are successfully combined with stock enhancement around New Zealand to enhance scallop yield per recruit, maintain a stable fishery and sustain full-time fishermen (Arbuckle & Metzger, 2000). On the Georges Bank grounds in the north-west Atlantic, four year closures resulted in 14-fold increases in scallop biomass (Murawski et al., 2000). Modelling studies have also indicated the potential benefits of spatial management for scallop fisheries (e.g. Caddy & Seijo, 1998; Hart, 2003). Myers et al. (2000) demonstrated that rotational harvesting of scallops is a robust management strategy in the presence of uncertainty about levels of indirect fishing mortality inflicted by dredging.

Aside from the effects of fishing on the target species, scallop dredging has also been shown to have lasting impacts on benthic environments and communities. Studies in the Irish Sea have demonstrated unequivocally that dredging alters benthic communities in both the short- and long-term (e.g. Bradshaw et al., 2000b; Brand, 2000; Veale et al., 2000). These studies have included comparisons between fished areas and a voluntary closed area, and the results suggest that spatial management of fisheries may be a valuable tool in the conservation of benthic habitats and communities.

The aim of this project is to provide frameworks for evaluating how permanent, seasonal and rotational closure of scallop fisheries around the British Isles could operate to ameliorate the impacts of commercial dredging on both benthic communities and the scallop stocks themselves. A spatially-structured scallop stock and fishery simulation model is developed that will allow the outcomes of spatial and other management to be evaluated in terms of scallop fishery yield and stock biomass. Information on benthic impacts of scallop dredging and likely time-scales for recovery is collated to assess the likely applicability of spatial management for protecting benthic ecosystems, and a framework is developed for evaluating the success of future schemes of scallop ground closures.

Objectives as set out in CSG 7

(1) Collate existing information on scallop biology and fisheries for parameterisation and validation of a simulation model.

(2) Construct a spatially structured simulation model of scallop populations and fisheries to evaluate stock and fishery response to management in terms of fishery yield, CPUE, spawning stock biomass and reproductive output.

(3) Collate and assess existing information on the utility of permanent, seasonal and rotational closed areas for the protection of benthic diversity, community and habitat structure.

(4) Collate and assess existing information on methodologies for the future evaluation of closed areas as practical measures to sustain scallop stocks and fisheries, while protecting benthic ecosystems.

Methods and Results

The following sections summarise the work and findings of this project under two main headings: the scallop simulation model, relating to Objectives 1 and 2; and benthic impacts, relating to Objectives 3 and 4. Fuller accounts of the project work are set out at Annexes 1 and 2 respectively.

Scallop simulation model

This section describes the development of a spatially structured simulation model of a scallop stock (Objective 2). The purpose of the model is to simulate stock dynamics and exploitation in order to assess the potential benefits from spatial and other management measures in terms of spawning stock biomass (SSB) and sustainable yield. The model is similar to that of Caddy & Seijo (1998), differing principally in the spatial allocation of fishing effort, the way in which mortality is calculated and resolved into its different components, the inclusion of spatially variable population and fishery parameters and the facility to define and combine multiple management controls. The model is implemented in a Turbo Pascal® programme, SPATMAN. A schematic summary of the model is shown in Figure 1.