A.R. Brand

Scallop ecology: distributions and behaviour

Abstract This chapter discusses aspects of the ecology of scallops concerned with their distributions and behaviour. Geographical and local distributions of scallop populations are reviewed, concentrating on the commercial species. The geographical distribution of each species is described with, where possible, illustrations showing the location of the main fishing grounds. All scallops have aggregated distributions within their geographical range. Scallops distributions at different spatial scales are described, and absolute densities, the establishment and maintenance of spatial pattern, temporal and spatial variability, spatial differences in population parameters such as age structures, growth rates and breeding cycles, genetic variation, recruitment mechanisms and the extent to which populations on different grounds are self-sustaining are discussed. Temperature is the primary factor determining the overall geographical range of a species but it interacts with water depth, substrate type, currents, food availability, turbidity, salinity and the occurrence of competitors and predators in determining the local distribution. Scallops are unusual among bivalve molluscs as many of them can swim, using a type of jet propulsion. Post-larval scallops are byssally attached and some species retain the byssus throughout life. Others lose the byssus and become free-living, using swimming to control their orientation, recess into the seabed and escape from predators. The behavioural mechanisms associated with byssus attachment, orientations and movements affect adult distribution. The occurrence, mechanism and function of these types of behaviour in different species are discussed, together with changes during ontogeny. Swimming is predominantly an escape mechanism but may serve other functions. Scallops react to contact by a predator by closing the shell, jumping or swimming, three distinct responses of increasing intensity. There is a trade-off between the mechanical protection of a heavy shell and swimming ability. In most species, juvenile scallops are better swimmers than the adults. With growth, the thicker shell provides better protection against attack by predators but swimming capacity decreases. Adult scallops eventually reach a size where they are not easily consumed by their common predators, reducing the need for an escape strategy. Many scallops can differentiate between potential predators and harmless species: this minimises feeding interruptions and prevents energy being wasted making unnecessary escapes. Scallops swim for short distances, become rapidly fatigued and take a long time to recover after swimming. They are well adapted for making short distance escape reactions but not for extensive migrations. In areas of strong current flow swimming scallops are carried downstream. Downstream dispersals account for the majority of scallop movements but a few species may undertake limited migrations.

Spat Settlement of the Scallops Chlamys Opercularis (L.) and Pecten Maximus (L.) On Artificial Collectors

Scallops of the bivalve family Pectinidae are very valuable shellfish and the subject of various important commercial fisheries around the world. Most of these fisheries are at present dependent upon natural populations, which can fluctuate widely in abundance from year to year due to irregular recruitment and overfishing. In recent years there has been considerable interest in the possibilities of cultivating various scallop species and some success has been obtained in rearing larvae of Pecten maximus (L.) in the laboratory (Comely, 1972; Gruffydd & Beaumont, 1972). However, by far the most successful cultivation system developed for pectinids has been carried out for several years in Japan for Patinopecten yessoensis (Jay). Naturally produced spat settling on Collectors made from artificial materials are grown on to market size in cages in the sea or used to restock natural beds (Ito, Kanno & Takashashi, 1975; Querellou, 1975; Motoda, 1977).

Effects of hypoxia and body size on the oxygen consumption of the bivalve Arctica islandica (L.)

Abstract The relationship between oxygen consumption and body weight has been examined in Arctica islandica (L.). Large Arctica exhibit a high degree of respiratory independence under hypoxic conditions; like other species studied, respiratory independence increases markedly with increasing body size. Respiratory independence can also be modified by temperature and physiological condition and it is concluded that the division of species per se into oxygen regulators and oxygen conformers is not merited. These terms describe only the extremes of what is a variable capacity to maintain respiratory independence during hypoxia.

The role of scallop-dredge disturbance in long-term changes in Irish Sea benthic communities: a re-analysis of an historical dataset

Abstract Benthic community data collected between 1938 and 1950 by N.S. Jones were compared with modern samples from seven sites in the Irish Sea. Multivariate and univariate methods were used to compare community change over time and examine the possible impact of scallop dredging over the 60 year time period. A conservative approach to data analysis ensured that observed differences in faunal composition between time periods were not due to differences in sampling methodologies or taxonomic identification. The community composition changed at all sites, though to different degrees. The amount of change was related to how long a site had been fished, rather than fishing intensity. Mobile, robust and scavenging taxa have increased in abundance, while slow-moving or sessile, fragile taxa have decreased. Differences between historical and modern samples were greater than could be accounted for by the natural variability of the system (as indicated by spatial and temporal replication at three sites) and indicate real long-term change. This study emphasises that, in the absence of good-quality data series and experiments, the use of ‘fuzzy’ historical data is often the only possible way to judge long-term change and can yield valuable results.

The effect of scallop dredging on Irish Sea benthos: experiments using a closed area

A 2 km2 area off the southwest coast of the Isle of Man (Irish Sea) has been closed to commercial fishing with mobile gear since March 1989. This area was heavily fished for Pecten maximus (Linnaeus, 1758) prior to closure, and the seabed immediately surrounding the closed area is still one of the most heavily dredged in the Irish Sea. Two methods have been used to study the effect of scallop dredging on the benthos in this closed area and adjacent fished areas. Firstly, twice-yearly grab sampling of experimental plots inside and outside the closed area since 1995 has enabled comparisons of the benthic infauna and epifauna of experimentally dredged plots, undredged control plots and plots exposed to commercial dredging. Secondly, divers have carried out visual transect surveys of P. maximus numbers regularly since closure. Communities of experimentally disturbed plots have become less similar to adjacent undisturbed control areas and more similar to commercially dredged areas. At each sampling date, similarity between dredged samples was greater than between undredged samples. Since 1989, there have been increases in the mean numbers of P. maximus in the closed area. The age structure of the closed area P. maximus population is also different to that outside, with a higher mean age due to the presence of large, old individuals.These results present strong evidence that scallop dredging alters benthic communities and suggest that the closure of areas to commercial dredging may allow the development of more heterogeneous communities and permit the populations of some species to increase. A common problem with studying fishing disturbance is the lack of good control sites and this work also demonstrates the value of closed areas to scientific studies of demersal fishing.

Benthic disturbance by fishing gear in the Irish Sea: a comparison of beam trawling and scallop dredging

1.?The distribution of effort for the most frequently used mobile demersal gears in the Irish Sea was examined and their potential to disturb different benthic communities calculated. Fishing effort data, expressed as the number of days fished, was collated for all fleets operating in the Irish Sea in 1994. For each gear, the percentage of the seabed swept by those parts of the gear that penetrate the seabed was calculated. 2.? For all gears, the majority of fishing effort was concentrated in the northern Irish Sea. Effort was concentrated in three main locations: on the muddy sediments between Northern Ireland and the Isle of Man (otter and Nephrops trawling); off the north Wales, Lancashire and Cumbrian coast (beam trawling); the area surrounding the Isle of Man (scallop dredging). 3.?In some areas, e.g. between Anglesey and the Isle of Man, the use of scallop dredges and beam trawls was coincident. A comparative experimental study revealed that scallop dredges caught much less by-catch than beam trawls. Multivariate analysis revealed that both gears modified the benthic community in a similar manner, causing a reduction in the abundance of most epifaunal species. 4.? Although beam trawling disturbed the greatest area of seabed in 1994, the majority of effort occurred on grounds which supported communities that are exposed to high levels of natural disturbance. Scallop dredging, Nephrops and otter trawling were concentrated in areas that either have long-lived or poorly studied communities. The latter highlights the need for more detailed knowledge of the distribution of sublittoral communities that are vulnerable to fishing disturbance. ©British Crown Copyright 1996.

Biological control of fouling in suspended scallop cultivation

Abstract Fouling of scallop shells and cultivation nets by living organisms is costly to remove and can reduce scallop growth. Here we investigate biological control of fouling in suspended scallop ( Pecten maximus ) cultivation. In preliminary trials in the Irish Sea, off the Isle of Man, sea urchins ( Echinus esculentus and Psammechinus miliaris ) and hermit crabs ( Pagurus spp.) removed fouling from nets more efficiently than did a range of other invertebrates. Sea urchins and hermit crabs were subsequently deployed from August 2000 to January 2001 at various densities (hermit crabs and E. esculentus at 1, 2, or 3 per net and P. miliaris at 3, 5 or 7 per net) in pearl nets containing scallops. After 6 months, the survival of biological control organisms and their effect on scallop growth and mortality, and fouling of nets and scallop shells, were assessed. Only P. miliaris was associated with increased scallop shell growth, but no biological control organism reduced scallop growth or survival. All three biological control organisms significantly reduced the weight of fouling on nets (often by as much as 50%) and fouling of scallop shells; results were largely independent of biological control organism density. Sea urchins were most effective, removing hydroids and solitary tunicates efficiently; they could thus be commercially exploited alongside scallops in a form of polyculture. These results suggest that biological control could be an efficient and environmentally sound method of addressing the problem of fouling in scallop cultivation.

The cardiac responses of the scallop Pecten maximus (L.) to respiratory stress

Heart activity of Pecten maximus (L.) has been recorded during various forms of experimentally induced respiratory stress. There was considerable variation in the responses of individual scallops but bradycardia generally occurred in response to all forms of respiratory stress, with the rate of fall in heart rate dependent upon the severity of hypoxia. When oxygen tension declined slowly in a closed respirometer there was regulation of both heart rate and oxygen consumption. The critical tension, Pc, for oxygen consumption lay between 70 and 80 mm Hg, and corresponded with a slight regulatory upswing of the heart rate, whereas the Pc for heart rate was much lower at 20–30 mm Hg. Sudden transfer to deoxygenated water for 3 h resulted in very rapid bradycardia and there was a rapid recovery and initial overshoot of the normal rate on return to well-oxygenated sea water. Aerial exposure for 3 h produced more gradual bradycardia followed by gradual recovery on return to sea water. The results of this work are compared in some detail with previous work on other species of bivalve from different geographical areas and habitats, and the mechanisms controlling cardiac and respiratory regulation are discussed. It is concluded that there are few clear-cut general differences between littoral and sublittoral species in their behavioural and physiological adaptations to hypoxia; the main distinguishing feature of littoral-adapted species is their ability to control air-gaping. Changes in heart activity generally indicate variations in metabolic rate, the speed at which the metabolic rate may be altered reflecting the degree of adaptation to the littoral environment.

An in situ study of predator aggregations on scallop (Pecten maximus (L.)) dredge discards using a static time-lapse camera system

The impact of demersal fishing gears on benthic habitats and species has been the subject of much attention recently, and suggestions have been made that scavenging epifaunal species may benefit at the population level from the additional food source provided by discards. This paper investigates some aspects of this process, including the relative attractiveness to predators of different discard species, and the role of damage in scavenger attraction. A time-lapse video system with a 1000 m long cable was positioned in an area closed to fishing, adjacent to the most heavily fished scallop (Pecten maximus) ground in the Irish Sea. A variety of undamaged and damaged by-catch animals were positioned in front of the camera, and the subsequent predator aggregations investigated. Densities of scavenger species up to 200 times that of the background population were observed, and aggregations of some species persisted for up to 3 days. The most frequently recorded scavengers, and therefore presumably those species most likely to benefit from discards as a food source, were: Asterias rubens L., Astropecten irregularis (Pennant), Liocarcinus spp Stimpson, Pagurus spp Fabricius and Callionymus lyra L. Predator attraction to apparently undamaged queen scallops, Aequipecten opercularis (L.), was almost as high as to damaged A. opercularis. Of all the prey species studied, queen scallops were the most attractive to scavengers. A directional relationship was found between the ambient water current and the arrival of the starfish, Asterias rubens.

Population dynamics and predictions in the Isle of Man fishery for the great scallop, Pecten maximus (L.)

There has been a fishery for the great scallop, Pecten maximus, around the Isle of Man, since 1937. Research up to the end of the 1980s suggested that these scallop stocks were in continuous decline. The fishery is now largely dependent on the strength of each recruiting year-class, placing it at considerable risk from recruitment failure. This study utilised data on the spat settlement, age structure, abundance and commercial catch rates of scallops, collected between 1975 and 2001, to examine recent population dynamics and the potential for predicting future catch rates. Spat settlement was generally low, but there were two exceptionally strong year-classes. Surveys of the stock revealed high exploitation rates during each fishing season (November to May inclusive) with variable recovery due to recruitment by the following October. In 1997/1998, scallop catch rates reached a 20-year high on several grounds and have generally remained high since. The strong spat settlements in 1989 and particularly 1994 were largely responsible for recent rises in catch rates, although the maintenance of high catch rates between 1999 and 2001 has occurred despite poor spat settlement between 1995 and 2000. Within stock surveys, the density of 2-year-old scallops was generally an accurate predictor of the density of 3- and 4-year-old scallops, 1-2 years later. The nature and strength of these relationships varied considerably between fishing grounds due to spatial variation in both scallop biology and patterns of exploitation. Results from fishery independent surveys did not always correlate well with commercial catch rates, however, suggesting the need for an expansion of the survey on some grounds. Overall, our study indicated that current levels of exploitation appear to be sustainable in the Isle of Man scallop fishery. Our results also demonstrated that monitoring of both spat settlement and the abundance of juveniles has considerable potential for predicting future catch rates of commercial sized scallops.