Norway lobster dynamics in areas closed to fishing – 20/02/09
Concern about the sustainability of many lobster fisheries.
Many loving couples will have feasted on langoustine, a type of lobster, to celebrate St Valentine’s Day, but there is great concern about the sustainability of many lobster fisheries. Intuitively, one might think that banning lobster fishing in designated ‘closed areas’ would be beneficial in the long term. But new research suggests that things are no so clear-cut.
Closed areas, in which fishing is prohibited, have been advocated for fishery management. The exclusion of trawlers should allow the population to recover within the protected zone, and stock in adjacent fished areas should also benefit, through enhanced reproduction and emigration. The benefits should, it has been argued, be particularly apparent for relatively sedentary animals like lobsters, the larvae of which are dispersed by currents.
Antony Jensen of the National Oceanography Centre, Southampton, and Philip Smith of the University Marine Biological Station, Millport, focus on the Norway lobster (Nephrops norvegicus) - langoustine, Dublin Bay prawn or scampi - which thrives on muddy seabeds on the Northeast Atlantic continental shelf, typically at depths of 15–500 m.
The Norway lobster fishery is one of the most valuable in Europe, worth approximately190 million Euros with some 60,000 tonnes landed annually. Norway lobsters are caught by trawlers, but also using baited traps. Because adult females are less active than males, leaving their burrows less often, fewer of them are caught, making the fished stocks more resilient.
The researchers assessed the potential for larval subsidy from the closed area to enhance the adjacent fishery using a mathematical model incorporating ecological information from the economically important and well studied Norway lobster fishery of the Firth of Forth in eastern Scotland.
Computer simulations with no closed area indicated that the yield of males could not be improved substantially by varying fishing effort from the long-term average, but that the yield of females could be increased somewhat with greater effort. Thus, the Norway lobster stock can withstand high levels of fishing effort.
But reduced fishing could increase the commercial value of the catch, increasing the average size of lobsters caught and the price commanded. An increase of 24% in the catch value was attained with a 55% reduction in fishing effort. However, the researchers highlight the need for an economic study of the factors influencing demand in Norway lobster markets and its relationship with price.
Simulated closure of part of the fishing grounds did lead to a long-term increase in total biomass of lobsters in the protected area, increased larval dispersal from the area, and increased recruitment and lobster numbers in the fished zone.
But the overall fishery yield was reduced. Fishing effort increased in the fished zone, reducing the average size and commercial value of lobsters caught. Moreover, say the researchers, “more-intense exploitation of small lobsters resulting from a closed area would lead to greater destruction of lobster biomass through discarding, which is undesirable ecologically, economically, and ethically.”
Implementation of a closed area also led to periods of boom and bust lasting many years after the closure, particularly when the closed areas were large and fishing effort was high. “It is not at all clear that a closed area would be economically beneficial in the long term”, said Dr Jensen, “in fact the value of the catch might actually fall.”
This is in line with studies of spiny lobster (Jasus edwardsii) fisheries around Tasmania and the coastal waters of Victoria, Australia, which have suggested that protected areas may not always be beneficial. However, the model is intended to illustrate the form of response to a closed area in a generic Norway lobster fishery, rather than to predict the consequences of an area closure in the Firth of Forth fishery specifically. This would require more explicit modelling.
Dr Jensen said, “the apparent resilience of Norway lobster stocks does not as justify less stringent fishery control measures. Norway lobsters living in deep water off the Iberian Peninsula have been overexploited, and we do not want to see the same thing happen in the Firth of Forth, or elsewhere.”
In fact, there are still many uncertainties about the mortality, growth rates, and movements in unfished Norway lobster populations. Such information would be valuable in modelling the dynamics of the adult stock with greater realism. Even cannibalism might be significant at high population densities.
The researchers point out that it is likely that, “faced with a reduction in yield, fishers would initially increase their fishing effort, if technically possible and legally permissible, in addition to targeting the most profitable areas.” They fear that, even setting aside the possibility of illegal fishing within the closed area, this would tend to exacerbate overfishing, discarding, and ecological impacts within the fished zone.
Their study, ‘Dynamics of closed areas in Norway lobster fisheries’, is published by the ICES Journal of Marine Science, and was funded by the European Union.
http://icesjms.oxfordjournals.org/cgi/content/full/65/9/1600
Image: Nephrops norvegicus, Cetaquarium, Crete, Hans Hillewaert
