In recent years, there has been considerable interest in the use of fishery exclusion zones (FEZ), or closed areas, as a fishery management measure. Closing an area to fishing is expected to result in increased biomass and greater average size of spawners within the protected zone. In turn, this may enhance the stock in the remaining fished zone, by increased reproduction and emigration of larvae, juveniles or adults. These benefits are most likely to be manifested in species with restricted mobility as adults and planktonic larval dispersal, as found in many marine invertebrates, such as lobsters.
There is some empirical evidence from existing closed areas within lobster fisheries of the expected increase in population density, average size and/or biomass within the protected zone. There is little information about the effects on recruitment and yield in adjacent fished zones, however. These effects are difficult to assess in the real world, because of confounding factors and lack of adequate spatial and temporal controls. Modelling provides a means of investigating the potential effects of different forms of fishery exclusion zone.
The aim of the present study was to investigate the possible effects of fishery exclusion zones in Norway lobster (Nephrops norvegicus) fisheries. This was part of a larger European Union-funded project (VALFEZ) assessing the biological and economic potential of fishery exclusion zones in a range of European fisheries. A dynamic, age-structured population model was developed, using biological and fishery information determined by the ICES Working Group on Nephrops Stocks for a Norway lobster fishery in eastern Scotland. A feature of the model was the incorporation of a domed stock-recruitment relationship, implying reduced recruitment at high spawning stock densities, as indicated by empirical data. Planktonic larvae were assumed to be dispersed evenly prior to settling to the seabed and movement of benthic phase lobsters between zones was assumed to be negligible.
A simulated Norway lobster population was modelled (separately for males and females) for 50 y prior to the introduction of a FEZ and for 100 y thereafter. Simulations were run for various combinations of FEZ size and prior fishing effort. After the introduction of a FEZ, the total fishing effort was not changed, but was concentrated within the remaining open area and fishing mortality there was adjusted accordingly.
The introduction of a fishery exclusion zone led to an initial rapid increase in stock biomass within the FEZ (Fig. 1). Survival, average age and size within the FEZ increased because total mortality rate was reduced to the rate of natural mortality only. FEZ biomass peaked after a few years, however, and then oscillated before stabilising at a level somewhat higher than the pre-FEZ value, but considerably less than the early maximum. In the open zone, biomass fell initially (owing to the increased fishing intensity there), before oscillating and stabilising at a level slightly less than pre-FEZ values. A FEZ led to a long-term increase in total stock biomass, though with larger FEZs, oscillations in biomass had not stabilised after 100 y.
Figure 1. Changes in biomass in open and closed areas in response to the introduction of a fishery exclusion zone 20% of the size of the original fishing area.
After the introduction of a FEZ, recruitment to the open zone increased, owing to the combination of greater total spawning stock biomass and reduced population density within the open zone itself (Fig. 2). With larger FEZs, recruitment in the open zone oscillated widely and had not stabilised after 100 y.
Figure 2. Changes in recruitment at age 1 in the open area in response to introduction of a fishery exclusion zone (size as a percentage of the original fishing area).
With the assumption of negligible movement of adult lobsters, enhanced recruitment did not offset the loss of part of the fishing area and fishery yield was reduced (Fig. 3). Larger FEZs led to a greater reduction in yield and, depending on the prior level of fishing effort, led to large oscillations in yield. Furthermore, the concentration of fishing effort in the fished zone after establishment of the FEZ, combined with the higher level of recruitment there, meant that the simulated fishery mainly exploited recently-recruited, smaller lobsters, commanding a lower price. Value of the catch was therefore reduced, while the quantity of moribund, undersized lobsters discarded was increased.
Figure 3. Changes in fishery yield caused by a fishery exclusion zone (size as a percentage of the original fishing area).
Oscillations in stock biomass, recruitment and yield were a notable feature of the results, particularly with large fishery exclusion zones. These oscillations were generated by the characteristics of the stock-recruitment relationship. It appears that the sudden cessation of fishing in part of the fishery area can cause considerable instability over several years. These results also warn that rapid increases in biomass and recruitment in the first few years after introduction of a FEZ are not good indications of the long-term consequences.
The benefits of a FEZ in terms of long-term enhancement of spawning stock biomass and recruitment to the open zone were greatest under high levels of fishing effort. This suggests that FEZs may be useful for sustaining recruitment and restoring spawning stock biomass in fisheries where effort is not well controlled. On the other hand, if it were possible to reduce fishing effort, a FEZ would have little impact on recruitment to the fished zone.
The model discussed here represented a hypothetical Norway lobster fishery and incorporated several simplifying assumptions. The results should not therefore be taken to demonstrate what the effects of a FEZ would be in any particular fishery. There are several uncertainties about how Norway lobster populations, fishermen and markets in the real world would respond to the introduction of a FEZ. Nevertheless, such models allow us to explore the type of dynamics which might arise in different circumstances and are particularly useful for highlighting key areas of uncertainty warranting further practical research.
We are extremely grateful to Nick Bailey and Ian Tuck (Fisheries Research Services, Aberdeen) for making available unpublished information on the fishery on which the model was based. This project was funded by the European Union under the Fifth Framework Programme.
VALFEZ project partnership
- Centre for the Economics and Management of Aquatic Resources, University of Portsmouth, UK (CEMARE, Coordinator)
- Centre de Droit et d'Économie de la Mer, Université de Bretagne Occidentale, Brest, France (CEDEM)
- Istituto di ricerche sulle Risorse Marine e l'Ambiente, Consiglio Nazionale delle Ricerche, Castellammare del Golfo, Italy (CNR-IRMA)
- School of Ocean and Earth Science, University of Southampton, UK (SOES)
