Effects of seed size and density on growth, tissue-to-shell ratio and survival of cultivated mussels (Mytilus edulis) in Prince Edward Island, Canada

In suspended aquaculture, the size of mussel seed and the density at which these mussels are placed in socks are two important factors affecting mussel production. However, the effect of these two factors has yet to be examined simultaneously. We conducted two large-scale field experiments to assess the effect of seed size and initial density on growth, tissue-to-shell ratio and survival of blue mussels (Mytilus edulis) grown on longlines. Experimental mussel sites were set up during the autumn of two consecutive years in two bays on the north shore of Prince Edward Island, Canada. At each site, socks containing combinations of three seed sizes (small, medium and large) at two initial densities (low and high) were deployed and monitored over time. Small seed grew faster than large seed and often reached commercial size in the same time period. Furthermore, initial density generally did not affect shell growth. During the pre-spawning period following the autumn deployment, seed at low initial density had a higher tissue-to-shell ratio than seed at high initial density, especially for large seed. This effect of initial density disappeared in early summer. Early in the experiments (< about 10 months), mussel survival on socks was affected by seed size only: small seed generally had lower survival rates than larger seed. Late in the experiments (> 10 months), there appeared to be an interaction between seed size and initial density on mussel survival: survival of small seed generally decreased with increasing initial density, while survival of large seed was not affected by initial density. Since large seed tended to be packed at lower initial density than small seed, lack of crowding was likely responsible for the absence of density-dependent loss. Our study thus assessed different seeding strategies, and results were generally consistent across years and sites.     

Protective socking material for cultivated mussels: a potential non-disruptive deterrent to reduce losses to diving ducks

Predation of cultivated mussels by diving ducks can threaten the viability of mussel farms. Conventional scaring tactics have had limited success at deterring ducks from feeding on cultivated mussels, because of rapid habituation and 24-h feeding cycles of some waterfowl species. We tested a socking material containing a biodegradable protective layer against predation by diving ducks on experimental blue mussel (Mytilus edulis) aquaculture sites in Prince Edward Island, Canada. We also assessed the effect of this new socking material on mussel growth and survival. Results showed that the protective socking has the potential to reduce losses to ducks, but did not perform well for all mussel seed sizes used by the industry. The protective layer also did not biodegrade fast enough, trapping a portion of mussels inside the sock, affecting growth and survival of these mussels. Improvements are needed to make this solution effective for all mussel sizes used by the industry and to make its production more cost effective.     

Seasonal movements of Atlantic rock crab (Cancer irroratus Say) transplanted into a mussel aquaculture site

Approximately 73 000 Atlantic rock crab (Cancer irroratus) were captured in coastal waters and transferred to an estuarine mussel aquaculture site in Prince Edward Island, Canada, in an effort to control overgrowth by the invasive tunicate Ciona intestinalis. To assess whether transplanted crabs remained in the vicinity of mussel farms, acoustic tags were attached to 25 resident and 25 transplanted individuals (all males) and their movements were monitored for 1 year, from September 2006 to August 2007. Resident crabs tended to occupy (within 1.6 km) the middle estuary where the majority of mussel farms are located. They left this area during winter, possibly to reach upstream overwintering grounds, and most returned to the middle estuary in spring. In contrast, translocated crabs exhibited a marked downstream movement (<5.5 km distant) in autumn and winter and few returned into the middle estuary in spring. Results suggest that transplanting large numbers of additional rock crabs is not an effective means to reduce the effects of invasive tunicates on mussel farms.     

Association between longline design and mussel productivity in Prince Edward Island, Canada

The first objective of this study was to document the design of subsurface longlines used for the farming of blue mussels (Mytilus edulis) in Prince Edward Island (PEI), Canada; the second objective was to identify possible associations between longline design and mussel growth. In 2003, SCUBA divers visited 54 farms distributed in 16 culture embayments; they measured the spacing between longlines and the spacing between individual socks (sleeves) attached to longlines; they also sampled one-year-old mussels for the determination of the shell length, condition index, and sock weight. Here we report a remarkable degree of variation in longline configuration, with the following range of values: 1.2 to 3.0 m for sock length, 1.5 to 29.5 m for longline spacing, 26.4 to 62.4 cm for sock spacing, and 6.2 to 179.9 socks/100 m² for stocking density at the farm scale. A negative correlation was found between stocking density at the farm scale and total farming area at the bay scale, suggesting that growers adjusted husbandry in relation to the surrounding level of farming activity. In one major culture bay, Tracadie Bay, measurements were repeated over a three-year period (2002-2004). This monitoring led to the discovery of a coordinated effort by growers in raising the average sock spacing by 30% (+ 11 cm). Multiple regression analyses identified sock spacing as the only explanatory variable correlated with mussel weight in Tracadie Bay. The model suggests that an 11-cm increase in sock spacing can lead to an 18% weight gain for pre-market mussels (∼ 34 mm), the size group investigated in the study. However, this correlation between sock spacing and sock weight was tenuous over the three-year study period, showing up only in 2002. A similar correlation was found between sock spacing and condition index, although only in 2004. We conclude by suggesting that close spacing of mussel socks can negatively affect mussel yield, although only under certain conditions.     

Biodeposit production and benthic loading by farmed mussels and associated tunicate epifauna in Prince Edward Island

An experimental study was done to evaluate the biodeposition dynamics associated with mussels and two fouling tunicates, Ciona intestinalis and Styela clava, in mussel aquaculture in Prince Edward Island (PEI), eastern Canada. The presence of C. intestinalis on small constructed mussel socks increased biodeposition by a factor of about 2 relative to mussel socks without tunicates. S. clava were small and had a negligible effect on total biodeposition from mussel socks although they increased sedimentation rates relative to that of abiotic control socks. Sinking rates of faecal pellets from large C. intestinalis varied between 1.39 and 6.54 cm s^− 1 (LSMean = 2.35 cm s^− 1). Using biodeposit production and sinking rates and hydrological data obtained in the present study, footprints of benthic loading due to mussel and tunicate biodeposition for a typical mussel farm in PEI were modelled using Shellfish-DEPOMOD. The results show benthic loading below longlines with C. intestinalis to be ca. 2 times greater than those from lines with only mussels with rates of up to 15.2 g m^− 2 d^− 1. However, given the greater settling rate of C. intestinalis biodeposits relative to mussel biodeposits, the extent of the footprint (≥ 1 g m^− 2 d^− 1) is similar or even more restrained.     

Field and laboratory transmission studies of haemic neoplasia in the soft‐shell clam,Mya arenaria,from Atlantic Canada

A two‐year laboratory and field study was initiated in 2001 in response to mass mortalities associated with haemic neoplasia (HN) in 1999 in Prince Edward Island (PEI) soft‐shell clams, Mya arenaria. A laboratory proximity experiment (cohabitation) and an inoculation challenge were conducted with clams and mussels (Mytilus edulis). Three field exposure experiments were also conducted, in which naive clams were held in sediment (in trays) or out of sediment (in mesh bags) at three high HN prevalence sites on PEI. There was a conversion to HN positive in clams in the proximity experiment and in clams injected with whole blood and cell‐free homogenate, but not at statistically significant levels. No mussels or control clams became HN positive. There was a significant conversion to HN positive in as little as 24 and 58 days after transfer with clams held out of sediment and in sediment, respectively. The laboratory and field experiments’ results suggest that HN‐infected clams are spreading the disease through water from infected clams to naïve individuals and via transplantation from affected to unaffected sites. Some environmental conditions (e.g. abnormally high water temperature and hypoxia‐induced sea lettuce [Ulva lacteus] invasion) may make clams susceptible to infections or exacerbate the proliferation of HN.     

Chlamydia-like organisms in the digestive diverticula of the bay scallop, Argopecten irradians (Lmk)

Abstract. The morphology of a chlamydia-like organism from the digestive diverticula of the bay scallop, Argopecten irradians (Lmk), is described. This organism was found in about 40% of bay scallops introduced to the quarantine unit at Ellerslie, Prince Edward Island, Canada and also in about 40% of the F₁ and F₂ generations. Procaryote infections were also found in the northern quahog, Mercenaria mercenaria (L.), the soft-shelled clam, Mya arenaria L., and the Atlantic deepsea scallop, Placopecten magellanicus (Gmelin).     

Evaluation of methods for assessing the retention of seed mussels (Perna canaliculus) prior to seeding for grow-out

The process of catching and relaying wild seed green-lipped mussels, Perna canaliculus, onto grow-out farms in New Zealand is extremely inefficient with the majority of the seed mussels being lost after transfer, mostly due to secondary settlement behaviour of the seed. In an attempt to identify the causes of this behaviour the retention of seed mussels was experimentally determined after exposure to three sub-lethal stressors, desiccation, starvation, and temperature fluctuation. In addition, two types of mussel seed behaviour were tested for their potential for developing a commercially useful method for identifying seed mussels with high retention properties prior to seeding. The two behaviours were the ability of seed mussels to attach by byssus threads to the substrate in slow flowing water, and the ability of seed mussels to pedal walk vertically up through a substrate filled column. Of the three stressors tested only desiccation and starvation significantly reduced mussel seed retention compared to “unstressed” controls by 38% (P < 0.05) and 42% (P < 0.05) respectively. Of the two types of mussel seed behaviour used for attempting to sort seed mussels, only the ability of seed mussels to attach by byssus threads to the substrate in slow flowing water showed any sensitivity for identifying individuals with higher retention properties (P < 0.05). Seed mussels that failed to byssal attach had a 23% lower mean retention, after 10 d experimental grow-out compared to seed that successfully attached. Although significant, this difference was relatively small and is unlikely to provide the basis for the development of a useful commercial seed mussel sorting method. Desiccation and starvation are both stressors that seed mussels are likely to experience during their commercial harvesting, transportation and seeding process. Consequently, seed mussels need to be relayed quickly and under moist conditions to reduce the potential for starvation and desiccation stress. Identifying seed mussels in poor nutritional condition and feeding them prior to seeding out on farms could also help to prevent the intermittent large scale seed mussel losses currently experienced. The application of these findings will assist in making more efficient use of the wild seed mussel resource and significantly lower financial losses to growers in New Zealand.     

Distribution of haemic neoplasia of soft‐shelled clams in Prince Edward Island: an examination of anthropogenic factors and effects of experimental fungicide exposure

Haemic neoplasia was first considered a disease of concern for soft‐shell clams in Prince Edward Island (PEI) when it was diagnosed as the cause of mass mortalities in 1999. The aetiology of the disease remains elusive, but has been associated with environmental degradation. In this study, a 2‐year (2001–2002) geographic and seasonal survey was conducted for haemic neoplasia, using histology, in soft‐shell clams from PEI. In addition, using geographic information system, the association between anthropogenic factors in the watersheds at sites affected by haemic neoplasia and the prevalence of the disease was investigated. Finally, histopathological changes were assessed in soft‐shell clams experimentally exposed to four concentrations of chlorothalonil for 27 days. Haemic neoplasia could not be induced at any concentration of chlorothalonil. Clams exposed to a concentration of 1000 μg L^?1 of the fungicide, however, exhibited an LC50 of 17 days. Although this information provides additional toxicity information (LC50) for soft‐shell clams, further experiments are required to assess longer term exposure to the fungicide. The highest prevalences of haemic neoplasia in PEI were found in North River and Miscouche (28.3–50.9% and 33.0–77.8%, respectively). No clear seasonal patterns were found. There was a correlation between haemic neoplasia prevalence and watersheds with a high percentage of potato acreage and forest coverage (P = 0.026 and P = 0.045, respectively), suggesting a link between anthropogenic activity and the prevalence of the disease.     

Lobster larval transport in the southern Gulf of St. Lawrence

A bio‐physical semi‐Langrangian model was developed to follow the drift and abundance of lobster larvae from hatching to settlement as post‐larvae. The geographic domain encompasses Northumberland Strait and the areas surrounding Prince Edward Island in the southern Gulf of St. Lawrence. The model was run for ten larval seasons, 1983–87 and 1997–2001, representing two periods of egg production. The model was forced with tides, winds, heat fluxes, and marine and freshwater fluxes at open boundaries. Biological inputs were location, date, and density of larvae at hatching, development rates, a time window and a minimum bottom temperature required for successful settlement, and two scenarios of daily mortality. Net drift was west to east but stronger on the north than the south side of Prince Edward Island. The hatch was greater in the second 5‐yr period, but the spatial patterns of settlement were similar. For both mortality scenarios, the same five of the 24 larval source areas were important in providing settlers. Horizontal shear of larval distribution indicates fishing communities were dependent on the hatch from fishing grounds of multiple upstream communities. Variation in daily and annual post‐larval settlement was greatest north of Prince Edward Island. From the 24 source areas there was an eightfold range in the fraction of larvae surviving to settlement, with advection into deep water an important cause of mortality. Four to five possible fishery management areas were identified using multidimensional scaling to group sink areas having shared source areas. We hypothesize leaky discontinuity in connectivity between these groups.     

Settlement of Mussel Perna perna Seed on Artificial Collectors Deployed on Sea Surface

The low settlement of mussel Perna perna seeds on artificial collectors deployed on sea subsurface has been a major obstacle to the mussel farming improvement in Brazil. This study aimed to compare the seed settlement on artificial collectors deployed on the sea surface with the aid of buoys and at the subsurface (20 cm depth) in Cocanha Beach, Caraguatatuba City, Sao Paulo State, Brazil. Mann–Whitney test showed significant differences (P < 0.05) between treatments for each class of length, with the best settlement occurring on the sea surface collectors for all classes (P < 0.05). It is concluded that deploying artificial collectors with buoys on the sea surface increases the number and the biomass of settled mussel seeds when compared to the non‐buoys deployed collectors on the sea subsurface.     

Biofouling on Greenshell? mussel (Perna canaliculus) farms: a preliminary assessment and potential implications for sustainable aquaculture practices

Greenshell? mussel (Perna canaliculus) culture is the primary aquaculture industry in New Zealand. However, our knowledge of biofouling on Greenshell? mussel farms, and its contribution to farm ecotrophic effects, is poor. We conducted a preliminary study of biofouling accumulation at two Greenshell? mussel farms during Intermediate and Final seed on-growing stages (each of 6?months duration) with sampling of mussel ropes at 0, 3, 5 and 6?months during each on-growing stage. A diverse range of biofouling organisms (71 distinct taxa) accumulated on mussel ropes, with biofouling biomass dominated by suspension-feeding organisms (~88% of biofouling biomass) such as other bivalves, ascidians and, to a lesser degree, bryozoans. Biofouling biomass increased with culture time, varied between farms and was generally greater at 2?m than at 8?m depth. After 6?months, biofouling organisms on average comprised 54% of the total rope biomass. The reseeding of ropes between Intermediate and Final seed crops reduced the amount of non-Greenshell? mussel biofouling. However, after 6?months, non-Greenshell? mussel biofouling on average still comprised 15% of the total rope biomass. To evaluate potential ecotrophic effects of biofouling on Greenshell? mussel farms, we compare the clearance rates of Greenshell? mussel longlines based on Greenshell? mussels alone and when combined with the two dominant biofouling species observed in our study (the mussel Mytilus galloprovincialis and the ascidian Ciona intestinalis). Our study shows that accumulated biofouling biomass on Greenshell? mussel ropes can be significant and recommends further investigation as to actual ecotrophic effects of biofouling to ensure sustainable mussel farm practices.     

Influence of densities on the growth of brown mussel Perna perna L. cultivated in suspended socks at Caraguatatuba,southeastern Brazil

This study was carried out in a marine area at Cocanha Beach, Caraguatatuba, southeastern Brazil, aiming to determine the most appropriated density for brown mussel (Perna perna L.) cultured in suspended socks (French System), in order to reduce costs with acquisition of juvenile mussels (seed) and labor. In September 2007, 15 socks measuring 2 m each were reared at densities of 300, 400 and 500 seeds/m. After 250 days, mussels were harvested and separated in two length classes: non-commercial (<6 cm) and commercial (≥6 cm). Mussels were counted and weighed. In August 2008, the experiment was repeated with 1-m-long socks at lower densities (200, 250 and 300 seeds/m) in order to identify differences among commercial-sized mussels only. Data were analyzed by ANOVA or nonparametric Kruskal–Wallis test, followed by Tukey or SNK tests, respectively. The results suggest that it is feasible to use densities from 250 to 300 seeds/m, since biomass yields are similar to those registered at higher densities and the use of mussel seed is therefore optimized.     

Chemical cues promote settlement in larvae of the green-lipped mussel, Perna canaliculus

The large (70,000 tonnes) mussel aquaculture industry for the endemic New Zealand green-lipped mussel, Perna canaliculus Gmelin, currently relies almost entirely (80%) on wild caught seed that is often harvested attached to great quantities of floating algae. Natural vagaries in catches of wild mussel seed frequently result in shortages of mussel seed that, at times, have severely affected commercial aquaculture production. Both field and laboratory experiments were used to establish that chemical cues derived from algae significantly increase settlement of mussel larvae on artificial substrates. This is the first time an algal chemical cue has been implicated in the settlement behavior of P. canaliculus larvae. These results have potential commercial implications for improving mussel seed supply through inducing higher or more reliable levels of mussel settlement for aquaculture seed collection.     

The effects of exposure and crawling behaviour on the survival of recently settled green mussels (Mytilus viridis L.)

The tolerance of recently settled spat of the green mussel (Mytilus viridis Linnaeus) to exposure was investigated. The survival rate was lower with longer periods of exposure. The effect of exposure on spat survival is discussed. Two types of crawling behaviour exhibited by the mussel spat were observed. The value of this behaviour to the animal's survival is discussed. The optimal tidal levels for spat settlement in Singapore waters were determined.     

Missing claws in a commercial catch of the American lobster: Implications for life history and the fishery

In most crustaceans, including the American lobster (Homarus americanus), a high frequency of individuals with missing claws is often reported. As lobsters with missing claws (“culls”) are less valuable in terms of weight and market value than uninjured lobsters, returning of culls to the sea has been suggested as a conservation strategy. To assess the potential benefits of such strategy, the frequency and types of claw loss were documented during the fishery season of 2007 (34 days, 6900 trap hauls) in an area of Prince Edward Island, southern Gulf of St. Lawrence. On average, culls accounted for ～7% of the total catch in weight and such percentage remained fairly constant along the study period. The raw frequency of culls, however, was higher in the first 9–12 days of the study and decreased progressively afterwards, reflecting the pattern followed by actual total catches. A single missing claw (pincher or crusher in similar proportions) was the most common type of injury for both males and females, followed by the presence of a single regenerating claw. The male:female ratio for the culls (～1:2) did not deviate from the one estimated for uninjured lobsters of the same size range (canners). These results are used to further discuss life history implications and the potential benefits and limitations of a conservation strategy based on the return of culls to the water.     

The role of shore crabs and mussel density in mussel losses at a commercial intertidal mussel plot after seeding

Mussel losses peak after relaying seed on culture plots. The present paper is an attempt to examine the role of shore crab predation and initial mussel density on mussel losses in mussel bottom culture using an intertidal culture plot as a case study. Because of their small size and loose attachment, mussels are particularly vulnerable to predation at this stage, as well as to handling stress and intraspecific competition. In the experimental field plots (1 × 1 m) in the intertidal Oosterschelde, three different densities (1, 5 and 10 kg m^?2) of mussel seed are laid, with half of the experimental plots protected from predation by means of exclosures. Duration of the experiment was 5 weeks (August–September 2012) post-seeding. Protection was the major factor accounting for biomass production, followed by mussel density. Loss rates increased with mussel density, both in the exclosures and in the exposed plots. Losses in the exclosures with the lowest density were still 45 %. There are indications that handling stress prior to the start of the experiment played a major role in these losses. At the higher densities in the exclosures, losses increased to 72.1 % and were not significantly different between 5 and 10 kg m^?2. About one-third of the total loss (32.6 %) was attributed to shore crab predation. The number of shore crabs observed on the plots did not differ between treatments. Byssal thread development during the experimental period was followed and was found to be a slow process that was insufficient to protect mussels from crab predation at the sheltered experimental location.     

枸杞岛贻贝养殖水域碳氮磷分布格局

于2013年贻贝成熟期和幼苗期对枸杞岛贻贝筏式养殖水域营养分布和碳分布格局开展相关实验。结果表明:从生态健康角度,夏季成熟期的贻贝养殖造成养殖海域氮、磷营养盐浓度升高,尤其是无机氮浓度显著高于非养殖区(P<0.05),养殖区处于受磷限制性的重富营养化,对其水体健康产生一定压力;秋季幼苗期贻贝养殖使养殖海域氮、磷浓度均降低,尤其是无机氮浓度显著低于非养殖区(P<0.05),与夏季分布格局相反,对水体有净化作用。从生态效应看,贻贝养殖使表层水体中溶解无机碳浓度显著低于非养殖区(P<0.05),产生碳汇效应;同时,贻贝养殖使水体中溶解有机碳浓度显著高于非养殖区(P<0.05),由于丰富的氮营养盐,溶解有机碳处于不稳定态,为碳源。因此,成熟期与幼苗期贻贝养殖对其水域健康产生不同作用,养殖的碳汇、碳源还有待进一步的系统研究。     

Effects of dietary carbohydrate on growth of juvenile New Zealand rock lobsters, Jasus edwardsii

The commercial aquaculture of spiny lobsters is impeded by the development of effective practical diets, which is partly due to a lack of knowledge of the digestive capabilities and nutritional needs of these species. Carbohydrates have the potential to provide a low cost energy source in practical diets for spiny lobsters. This research investigated the effect of algal carbohydrates, on the growth of juvenile rock lobsters. Juvenile lobsters, Jasus edwardsii, (10-12 mm carapace length) were provided with one of five diets; blue mussel, squid (Nototodarus gouldii), or a mussel diet partially substituted with algal carbohydrates, either agar, carrageenan or alginate over 80 days in culture. In all treatments, diet consumption and lobster growth were maintained or increased over time. Growth was highest for the mussel diet followed by the agar diet. Similar lobster growth occurred on the carrageenan and alginate diets, with the least on the squid diet. After 80 d there were significant differences in the percent lipid and glycogen stored in the mid-gut gland with highest levels of glycogen in lobsters fed the mussel only diet. Percent protein in the tail muscle was greatest for mussel fed lobsters. The mid-gut index (mid-gut weight/whole wet weight) was highest for the mussel only diet and this was similar to control lobsters at the start of the experiment. The results indicate that glycogen from mussels and agar could be used as carbohydrate source in the future development of a practical diet for spiny lobsters.     

Population structure of Ctenochaetus striatus (Acanthuridae) in Tutuila,American Samoa: The use of size-at-age data in multi-scale population size surveys

We combined size-at-age and underwater survey (UVS) data to investigate the spatio-temporal distribution of life-history stages of the bristletooth surgeonfish, Ctenochaetus striatus, around Tutuila Island, American Samoa. Three age-based size categories (recruits, juveniles and adults) were determined by examining the relationship between size, age and gonad maturity. The results from combining size-at-age with UVS density data showed varying densities at the scales examined (years, exposure, sector, and habitat) on the fore-reef slopes of Tutuila Is. While recruit densities were uniform, juvenile densities varied by year, exposure and sector, and adult densities were significantly higher at topographic sites that were comparatively more exposed to water motion than inside bays. Densities of recruits were positively correlated with habitat (rubble) illustrating the importance of this substratum for the survivorship of early life history stages. Densities of adults were positively correlated with coralline algae, which may suggest co-occurrence responding to similar physical environment of adult habitat that is possibly indirectly correlated to feeding. However, densities of juveniles were negatively correlated with food source although the variance accounted for was low. We hypothesize that some other substratum may possibly be important for juveniles. There was a negative correlation between recruit and adult densities, indicating the presence of ontogenetic shifts in habitat with age and size. It is proposed that ecosystem-based fishery management of C. striatus in Tutuila Is should consider the high abundance of adults in exposed topographic habitats in the establishment of marine protected areas. We have shown that size-at-age data when combined with underwater fish surveys significantly expands its utility in the study of the population structure of a coral reef fish thus allowing the identification of areas characterized by high abundance of particular life stages for management.