Abstract
For the cultivation of mussels, wild stocks of juveniles are harvested to collect mussel seed as starting material for the culture. These wild stocks are found in the sublittoral western Wadden Sea (NL). After summer spat fall, fisheries in Autumn on newly formed beds is carried out in areas that have the risk of washing away due to storms, or are vulnerable for starfish predation. These wild beds are considered as relatively unstable. On remaining more stable wild beds, seed fishery is carried out in next Spring. As the Wadden Sea is a nature conservation area, mussel seed fisheries is only allowed if no negative impacts on the nature management objectives can be expected. Seed fishery impacts were addressed in an extensive study including effects on sediment composition, macrobenthos and epifauna. In this paper we describe the effects of mussel seed fisheries on the development of the mussel stocks with and without fisheries in 39 pairwise studied impact and control plots.Stocks on seed beds in areas of the sublittoral Western Wadden Sea that are known as unstable, show a large decline within one year after settlement, also when there is no seed fishery. Harvesting seed on more stable beds in Spring results in a statistical significant reduction in stock size, which lasts for a period of two years after the first fishery. For the longer term, there is a gradual decline of the mussel stocks on all studied plots. On three out of the 39 plots, mussel biomass showed a large increase, both on control and impact parts. Also these mussel beds declined and eventually disappeared. A difference in life expectancy of fished and unfished beds was not demonstrated.It is concluded that sublittoral beds gradually disappear, also without fisheries. As a consequence, new recruitment is of critical importance for the long-term survival of sublittoral mussel beds. As we found no significant difference between recruitment on fished and control parts, there are no indications for negative impacts of seed fishery on new recruitment.
Highlights
Bivalve shellfish culture is an extensive type of aquaculture
We address the effects of mussel seed fishery on the mussel stock dynamics
Fishing effort in the impact parts of the plots has been registered by the black box on board of the vessels. This is relevant in order to quantify the treatment and to establish if fishery has been carried out as usual in mussel seed fishery
Summary
Bivalve shellfish culture is an extensive type of aquaculture. There is no addition of feed or medicine, culture is carried out in the natural environment and in the traditional culture, the juveniles are collected from the environment. The dredges are constructed in such a way that the mussels are caught in the dredge, while the underlying sediment remains intact It can be questioned in how far this type of fisheries disturbs the bottom in such a way that it is detrimental to flora and fauna, to the development of the mussel beds and to the quality of the area for new recruitment (Dolmer et al, 2001; Dolmer, 2002; Ysebaert et al, 2009). Impact studies of mussel seed fishery in intertidal areas have shown effects of harvesting on the target species stock and subsequent limitation in food availability for foraging birds (Beukema, 1993; Nehls et al, 1997; Herlyn and Millat, 2000; Ens, 2006). Some papers report about the impact of subtidal mussel fishery (Buhs and Reise, 1997; Dolmer et al, 1999; Saurel et al, 2004), to our knowledge no control – impact studies have been published about the effects of seed fisheries on subtidal wild mussel beds
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