Some aspects of modelling scallop dredge efficiency and special reference to dredges with depressor plate

Journal of Shellfisheries Research, August, 2004 by Spyros Fifas, Joel Vigneau, William Lart

ABSTRACT To estimate the abundance of scallop populations either by VPA approach or by directed survey on seabeds it is necessary to evaluate the efficiency of fishing gears. The age-size structure of commercial or experimental catches does not usually correspond to that of the targeted population, Several previous studies on scallop stocks in Scottish or English Channel waters show that different functions can be fitted on relationships of dredge efficiency versus scallop size (monotonic increasing functions or curves including increasing and decreasing stages). The two ways to tackle the question involve to calculate efficiency estimates, absolute (by diving after dredge haul) or relative (by comparing two different parameter dredges in standardized sampling conditions). This study develops aspects of methodology applied on calculation of dredge efficiency parameters. The study focuses on data of English Channel French scallop fisheries.

KEY WORDS: scallop, dredge, depressor plate, efficiency, logistical curve, variance, Taylor's polynomial

INTRODUCTION

In the assessment of the abundance and biomass of scallop populations, there is a requirement to calculate the efficiency and selectivity of the fishing gear. An extrapolation has to be made because age-size composition of commercial or experimental catches does not correspond to that of the actual population. A multiplicative factor including tow components has to be taken into account: (1) the first component is a correction rate independent of scallop size because only a part of the individuals are retained by dredging and (2) the second factor depends on size because of selectivity component through meshes and tooth spacing (Dare et at. 1993, Fifas & Berthou 1999).

After several recommendations for general methodology, we report on an evaluation of dredge efficiency experiment either using divers (Saint Brieuc Bay, Western English Channel; Fig. 1) or using two different mesh size dredges (Bay of Seine; Eastern English Channel; see Fig 1). In both cases, the experimental fishing gears are dredges filled with a depressor plate (Fig. 2) and equipped with a belly and back ring size of 50 mm.

[FIGURES 1-2 OMITTED]

In the first case, the estimator is called absolute efficiency written as (Laurec & Le Guen 1981, Buestel et al. 1985):

(1) absolute efficiency = number of scallops in the dredge/total number of scallops (dredge sea bottom)

In the second case, we usually compare two different mesh size dredges (Vigneau et al. 2001). The experiment can also be performed between two types of vessels; Weinberg et al. (2000) compared swept area estimates of research and commercial vessels on catches of Atlantic surfclam (Spisula solidissima). The comparative estimator called relative efficiency is expressed by:

(2) relative efficiency = number of scallops in the first dredge (or vessel)/ number of scallops in the second dredge (or vessel)

Components of Efficiency

Dickie (1955), Dupouy (1982), and Dare & Palmer (1994) calculated dredge efficiency by tagging/recapture of scallops. Chapman et al. (1977) and Mason et al. (1979) worked on standard and spring-loaded dredges by using divers to survey the track after dredging; the same approach was also developed on dredges with depressor plate by Buestel et al. (1985); Beukers-Stewart et at. (2001) estimated absolute and relative efficiency of spring-loaded dredges. Similar studies were conducted on other species (e.g. oysters: Ostrea lutaria, Allen & Cranfield 1979; Tiostrea chilensis, Doonan et al. 1994; black scallop, Chlamys varia: Shafee 1979: blue crab, Callinectes sapidus: Zhang et al. 1993; Voelstad et al. 2000; blue mussel, Mytilus edulis: Dolmer et al. 1999). Fifas (1991) and Fifas and Berthou (1999) also adopted the diving method for European scallops and argued that dredge efficiency in the case of fishing gear equipped with depressor plate can often be fitted by increasing monotonic curves (logistical equation). Components of dredge efficiency were listed by Dare et al. (1993) for spring-loaded gears and by Vigneau et al. (2001) for gears with depressor plate. This review is completed by Horton (personal communication):

(1) Combination of vessel speed/warp length: combination of both parameters exert influence on dredge towing tension: to stabilize towing efficiency, the higher dredging speed the longer warp (e.g. warp length is usually equal to three times depth for a dredging speed of three knots).

(2) Water depth: Water depth potentially introduces an auxiliary variable of efficiency related to warp weight (depending on warp diameter and material) even if warp length is optimized against vessel speed. Potentially an increase of water depth reduces dredge penetration in sediment because the force exerted on warp in traction is the resulting vector of towing traction and warp weight (warp buoyancy can be neglected).

(3) Direction of tow in relation to tidal flow: experience of commercial skippers confirmed by diving operations during scientific surveys emphasizes the effect of tow direction against tide: it was conclusively proved that the direction of tow has to be directed against the tidal current of weak or mean coefficient (e.g. less than 90 in the Western English Channel) to reduce random variations of dredge efficiency.