Rock Crab Research Articles

WITHDRAWN: Nonparametric Mixture Analysis of Rock Crab of the Genus Leptograpsus

WITHDRAWN: Nonparametric Mixture Analysis of Rock Crab of the Genus Leptograpsus

Short- and long-term dietary effects on disease and mortality in American lobster Homarus americanus

The American lobster Homarus americanus fishery is heavily dependent on the use of fish as bait to entice lobsters into traps. There is concern that this food supplementation is nutritionally insufficient for lobsters, but previous experiments reported conflicting results. We conducted a long-term feeding experiment in which 1 yr old American lobsters were fed one of 7 diets for a period of 352 d, a time that allowed the lobsters to molt thrice. The diets consisted of fresh frozen herring, a 'wild' diet (rock crab, mussel, and Spirulina algae), a formulated artificial diet for shrimp, paired combinations of these 3 diets or a diet formulated at the New England Aquarium (Artemia, fish and krill meal, Spirulina algae, soy lecithin, vitamins and minerals). The lobsters fed the diet of 100% fish had higher initial molting rates, but within the period of this experiment all either contracted shell disease or died. Mixed diets resulted in higher survival and a lower probability of mortality. This research demonstrated a critical time component to diet studies in lobsters. Short- and long-term impacts of diet differ. In the long term, continual high consumption rates of fish by the lobsters promote poor health in all lobsters, not just those of market size. The use of fish as bait may make lobsters more susceptible to the stress associated with environmental fluctuation, thereby leading to increased disease and mortality. This nutritional stress can be used to develop a laboratory model of shell disease in American lobsters.

Skin Test for Paragonimiasis among Schoolchildren and Villagers in Namback District, Luangprabang Province, Lao PDR

As a part of a broader effort to determine the status of Paragonimus species infection in Lao PDR, an epidemiological survey was conducted on villagers and schoolchildren in Namback District between 2003 and 2005. Among 308 villagers and 633 primary and secondary schoolchildren, 156 villagers and 92 children evidenced a positive reaction on a Paragonimus skin test. Only 4 schoolchildren out of 128 skin test-positive cases had Paragonimus sp. eggs in their sputum, all of which was collected on 1 day. Several types of crabs, which were identified as the second intermediate host of the Paragonimus species, were collected from markets and streams in a paragonimiasis endemic area for the inspection of metacercariae. Among the examined crabs, only "rock crabs" (Indochinamon ou) harbored Paragonimus sp. metacercariae, and it is speculated that the life cycle of Paragonimus sp. was maintained via rock crabs in Namback District, Lao PDR.

Physiological Responses of Rock Crab Cancer Irroratus Exposed to Waterborne Pollutants

Abstract This study investigates the impact of waterborne xenobiotics on the rock crab Cancer irroratus. Male rock crabs were caged at two polluted sites for five weeks. One site was highly polluted while the other was slightly polluted mainly by metals. A control group was caged in a raceway tank supplied with clean sea water. The activity of branchial ATPases was increased in crabs exposed to pollution, presumably to compensate for the competition of metals with electrolytes and to facilitate the transport of metals out of the cytoplasm. The rock crab was characterized as having high basal levels of glutathione in the digestive gland (∼ 3500 nmol GSH equivalents/g dry weight). Exposure to waterborne pollutants failed to trigger an increase in glutathione concentration, but over time in crabs caged at the highly polluted site it elicited a 26% increment in the relative mass of the digestive gland which is an important site for glutathione synthesis. Neither cytochrome C oxidase, nor lactate dehydrogenase...

Spatially referenced trap arrays detect sediment disposal impacts on lobsters and crabs in a New England estuary

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 348:249-260 (2007) - DOI: https://doi.org/10.3354/meps07009 Spatially referenced trap arrays detect sediment disposal impacts on lobsters and crabs in a New England estuary Kerrie P. O’Donnell1,4,*, Richard A. Wahle2, Michael Bell3, Michael Dunnington2 1Island Institute, PO Box 648, 386 Main St., Rockland, Maine 04841, USA 2Bigelow Laboratory for Ocean Sciences, 180 McKown Point Road, West Boothbay Harbor, Maine 04575, USA 3The Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK 4Present address: University of British Columbia—Fisheries Centre, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada *Email: k.odonnell@fisheries.ubc.ca ABSTRACT: We employed spatially referenced trap arrays and side-scan sonar mapping of the sea floor to assess the impact of late autumn sediment disposal on local American lobster Homarus americanus and rock crab Cancer irroratus abundance in the neighborhood of a disposal site for harbor dredge spoils in Penobscot Bay, Maine, USA. The assumption that impacts on lobsters would be minimized if disposal were conducted in late autumn, when the commercial lobster fishery is at a seasonal low and lobsters are believed to be moving to deeper water, was tested. Previous mark–recapture experiments conducted nearby established that trap catch rates strongly track trends in abundance. To our knowledge this is the first study in which geo-referenced trap-based sampling and seabed mapping have been used to assess the impact of an environmental perturbation on mobile benthic megafauna. We set 3 arrays of 24 traps: 1 array over the disposal area and 2 on adjacent unimpacted areas, all about 0.6 km2 in size. Two weeks prior to the onset of disposal and for 4 wk thereafter, we counted lobsters and rock crabs caught in the traps weekly and released them at the same trap location. Post-disposal side-scan sonar surveys revealed a mound of soft sediment not evident in pre-disposal surveys covering an estimated 44170 to 108881 m2 of seabed. No statistically significant impact of the sediment disposal was detected on lobster abundance. In contrast, within a few weeks of the onset of disposal, the overall catch of rock crabs increased dramatically in the disposal area and specifically on the patch of newly deposited sediment. These results indicate that spatially referenced trap arrays used in conjunction with seabed mapping provide a useful tool for evaluating change in the distribution and abundance of benthic megafauna at spatial scales as small as 10s of meters. The findings also indicate that even for ecologically similar taxa, e.g. lobsters and crabs, the impact of such disposal events can be strongly species specific. While direct impacts to lobsters may be minimized after the autumn offshore migration, crabs, still abundant in the area, may be attracted to the food-rich sediments at the disposal site. KEY WORDS: Anthropogenic disturbance · Dredge material disposal · Traps · Homarus americanus · Cancer irroratus · Abundance Full text in pdf format PreviousNextCite this article as: O’Donnell KP, Wahle RA, Bell M, Dunnington M (2007) Spatially referenced trap arrays detect sediment disposal impacts on lobsters and crabs in a New England estuary. Mar Ecol Prog Ser 348:249-260. https://doi.org/10.3354/meps07009 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 348. Online publication date: October 25, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Molecular cloning of four cDNAs encoding prepro-crustacean hyperglycemic hormone (CHH) from the eyestalk of the red rock crab Cancer productus: Identification of two genetically encoded CHH isoforms and two putative post-translationally derived CHH variants

Recently, we demonstrated that the four known sinus gland (SG) isoforms of Cancer productus crustacean hyperglycemic hormone precursor-related peptide (Capr-CPRP I–IV) are differentially distributed in conserved patterns among individual crabs. This finding strongly supported the presence of multiple prepro-crustacean hyperglycemic hormone ( chh) transcripts in each crab, as well as the translation and processing of the encoded prepro-hormones. Whether these transcripts contained common or distinct isoforms of CHH remained unknown. To address this question, molecular analyses of the C. productus eyestalk prepro-chhs were undertaken. Using a PCR-based cloning strategy, four prepro-chh cDNAs were characterized: one encoding CPRP I, one encoding CPRP III (found to possess Ile 26 rather than Leu 26 as reported previously), and two encoding CPRP II. No cDNA encoding CPRP IV was identified. The deduced CHH present in the prepro-hormones containing CPRP I and III were identical (Capr-CHH I) and differed from that (Capr-CHH II) present in the two prepro-hormones containing Capr-CPRP II at a single residue, a Thr 5 for Ser 5 substitution. As both CHH isoforms possess Glu at position 1, a cyclization of this residue to pyroglutamine is likely as the peptides mature, as has been seen for the CHHs of other brachyuran species. Likewise, homology to other CHHs suggests all C. productus isoforms are C-terminally amidated. These post-translational modifications would result in four SG isoforms of CHH: Capr-CHH I, Capr-pyro-CHH I, Capr-CHH II, and Capr-pyro-CHH II. Southern blotting supported the hypothesis that at least three prepro-chh transcripts are present in each crab, while dual in situ hybridization-immunohistochemistry localized the transcripts to previously mapped CHH immunopositive somata in the X-organ, the major source of innervation to the SG.

Gastric processing and evacuation during emersion in the red rock crab, Cancer productus

The passage of a radio-opaque meal was followed through the digestive system of the red rock crab, Cancer productus, using a fluoroscope. When the crabs were maintained in seawater, the food was apparent in the foregut as soon as the animals had fed. Release of food from the foregut was routinely slow and digesta appeared in the midgut only in small amounts at any one time. The foregut was emptied between 24 and 36 h, digesta was cleared from the midgut region at 36 h and by 48 h only a small amount of residual digesta was left in the posterior part of the hindgut. Contractions of the cardiac region of the foregut were somewhat sporadic and ranged between 6 and 11 min−1. Contractions of the pyloric region were more stable, varying between 45–65 min−1. In both cases, there was no change in rate during 18 h period in seawater. When crabs were subjected to both short- and long-term aerial exposure, release of food from the foregut was halted for the first 4–6 h of emersion. Although, small amounts of digesta appeared in the midgut and hindgut, there was no significant change in the amount in each region during emersion. There was a trend towards a depression of cardiac stomach contraction rates, but this was only significant in 3 h postprandial crabs during short-term emersion. A pronounced decrease in pyloric stomach contraction rate was maintained for the duration of the aerial exposure. When crabs were returned to seawater, contraction rates took 3–5 h to return to normal, but no significant change in gastric evacuation was observed during this period. During re-immersion, over 65% of the animals regurgitated the stomach contents. This regurgitation may act as a protective mechanism to avoid digestion and the subsequent specific dynamic action. The decrease in gastric processing in C. productus is probably part of an overall metabolic depression occurring during emersion.

Eight-year Record of Hemigrapsus sanguineus (Asian Shore Crab) Invasion in Western Long Island Sound Estuary

Hemigrapsus sanguineus (Asian shore crab) first arrived at Rye, NY in 1994. The intertidal abundances of H. sanguineus, Carcinus maenas (green crab), and the native crabs Eurypanopeus depressus (flatback mud crab), Cancer irroratus (Atlantic rock crab), and Libinia emarginata (spider crab) were censused from 1998-2005. Asian shore crab densities (estimated in June) in- creased from 1998-2001 to ca. 120 crabs m-2, and then declined to 80 crabs m-2 from 2002-2005. The flatback mud crab declined in abundance by about 95%. Decreases in the abundances of Atlantic rock crabs, green crabs, and spider crabsmay also have occurred, though these species were uncommon at the outset of the study. The lower intertidal density of the gastropod Littorina littorea (common periwinkle) decreased by about 80%, and the decline was coincident with the expansion of the Asian shore crab population. In June, small Asian shore crabs were disproportionately more abundant in the upper intertidal zone com- pared with lower zones, where large crabs were more abundant. January intertidal populations were dominated by small Asian shore crabs, and these were restricted to the lower half of the intertidal zone.

AMP‐activated protein kinase (AMPK) affects temperature tolerance in the rock crab, Cancer irroratus

AMP‐activated protein kinase (AMPK) affects temperature tolerance in the rock crab, <i>Cancer irroratus</i>

Distribution, abundance, and feeding of a disjunct population of lady crab in the southern Gulf of St. Lawrence, Canada

The lady crab (Ovalipes ocellatus) is one of the most common native species of swimming crab (Portunidae) of the Atlantic Coast of North America but most populations occur south of Cape Cod, Massachusetts. There is a disjunct population in Northumberland Strait (southern Gulf of St Lawrence), which was the focus of this study. Adult lady crabs were collected by trawling in water >4 m deep from May to October 1999 to 2005 to determine abundance, distribution, and diet. Lady crab occurred only in a small area (about 2,500 km2) in the central part of Northumberland Strait where bottom water temperature was >18°C during summer, and the substrate was mainly sand or sandy gravel. Male lady crab attained a maximum carapace width (CW) of 112 mm compared to 92 mm CW for females. The summer and autumn diet consisted mainly of infauna. The principal prey (each >5% of diet by weight) were: small bivalve molluscs (primarily Atlantic razor clam Siliqua costata and Macoma sp.; 43%), small rock crab (Cancer irroratus; 13%), polychaetes (11%), fish remains (9%), and small lady crab (9%). All stomachs collected during May (water temperature ≤10°C) were empty. There was little evidence of any difference in feeding intensity between 0700 h and 1900 h.

SEDIMENT SELECTION BY JUVENILE SEA SCALLOPS (CANCER IRRORATUS SAY)

Abstract We examined sediment selection by juvenile sea scallops (Placopecten magellanicus) and their predators, sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus). In laboratory trials, groups of scallops (~30 mm shell height) were simultaneously offered four sediment types in the presence and absence of a sea star or a rock crab: (1) glass representing a homogeneous, hard bottom; (2) sand; (3) granule and (4) pebble. As well, individual predators were offered the four sediment types without scallops. The number of scallops and the proportion of time predators spent on each sediment type were monitored over time. When compared with the expected distribution, scallops avoided glass and tended to select granule and pebble sediments when alone. In the presence of a rock crab, scallops also avoided glass. However, scallop distribution was similar to the expected distribution when a sea star was present, because sea star encounters are an important trigger of scallop swimming, leading to frequent...

Sacculina nectocarcini, a new species of rhizocephalan (Cirripedia: Rhizocephala) parasitising the red rock crab Nectocarcinus integrifrons (Decapoda: Brachyura: Portunidae)

The parasitic barnacles, Rhizocephala, are a little known group within Australia with only seven described species from a coastline stretching approximately 59763 km. This study describes a new species of Rhizocephala, Sacculina nectocarcini. The description is based on a unique combination of features pertaining to the structure of the mantle papillae, the retinaculae and the male receptacles. Biological notes, prevalence and intensity of infection are reported for this rhizocephalan, infesting the red rock crab, Nectocarcinus integrifrons, collected from Western Port, Victoria, Australia.

BIOTIC AND ABIOTIC FACTORS INFLUENCING GROWTH AND SURVIVAL OF WILD AND CULTURED INDIVIDUALS OF THE SOFTSHELL CLAM (MYA ARENARIA L.) IN EASTERN MAINE

A series of intertidal field experiments was conducted from 1986–2003 in eastern Maine to examine biotic and abiotic factors influencing the growth and survival of wild and cultured individuals of the softshell clam, Mya arenaria L. Separate experiments examined: (1) the efficacy of transferring sublegal wild clams (<50.8 mm SL) from areas near the high intertidal zone where shell growth was slow to areas where growth was predicted to be faster; (2) effects of tidal height on wild and cultured clam growth; (3) effects of spatial variation on cultured clam growth; (4) dispersion and growth of cultured juveniles in small experimental units; (5) effects of the naticid gastropod, Euspira heros Say, predation on survival of wild and cultured clams and (6) the species composition of large, crustacean predators that forage intertidally during periods of tidal inundation. Protective netting (4.2 mm aperture) increased recovery rate of transferred clams by 120% and resulted in a 3-fold enhancement of wild recruits. Effects of tidal height on wild clam growth revealed complex behaviors in >0 y-class individuals. Clams growing near the upper intertidal take >8 y to attain a legal size of 50.8 mm SL, whereas animals near the mid intertidal generally take 4.5–6.5 y. Unexpectedly, clams initially 38–54 mm SL and growing near the extreme low tide mark at a mud flat in Eastport, Maine, added, on average, <2 mm of new shell in a year, which was 8–10 mm SL less than animals at higher shore levels. It is hypothesized that biological disturbance by moon snails, that consumed >90% of clams at the low shore levels, contributed to this slow growth. In another field trial from 1986–1987, moon snails and other consumers were allowed access to clams ranging in size from 15–51 mm. E. heros preyed on clams over the entire size range and attacked clams between 31–40 mm at a rate that was nearly double what had been expected. Mean snail size was estimated to range from 10–52 mm shell height (SH), based on a laboratory study that yielded information about the linear relationship between snail size and its borehole diameter. In an experiment from June to September 1993, moon snails consumed >70% of juvenile clams (ca. 10 mm SL) within a month after planting at each of three tidal heights. Snail sizes ranged from 15–20 mm SD with larger individuals occurring near the upper intertidal zone. Green crabs, Carcinus maenas (L.) also prey heavily on softshell clam populations, but most studies that use shell damage to assign a predator have assumed that all crushing and chipping predation is because of this invasive species. An intertidal trapping study demonstrated that both green crabs and rock crabs, Cancer irroratus Say, are present during periods of tidal inundation, with the latter species accounting for ca. 40% of large crustacean numbers.

Neuroendocrine regulation of carbonic anhydrase expression in the gills of the euryhaline green crab,Carcinus maenas

The relationship between branchial carbonic anhydrase (CA) activity, CA gene expression and salinity, and potential mechanisms of regulation, was investigated in the euryhaline green crab, Carcinus maenas, acclimated to 33 ppt and transferred to 10 ppt, and the stenohaline rock crab, Cancer irroratus, acclimated to 32 ppt and transferred to 18 ppt. CA activity in green crabs acclimated to high and low salinity was a function of CA mRNA expression, with low salinity exposure resulting in an increase in both CA expression and activity. Eyestalk ablation (ESA) in green crabs acclimated to high salinity resulted in an increase in CA expression in the posterior, ion-transporting gills, in the absence of the low salinity stimulus. There were no changes in CA activity or expression in the anterior, respiratory gills. ESA also potentiated low salinity-stimulated CA induction, again, only in posterior gills. There were no changes in CA activity in any gills of Cancer irroratus, in response to either ESA or low salinity. These results suggest that CA expression in euryhaline, osmoregulating species, is under inhibitory regulation by a putative repressor found in the eyestalk, and that this mechanism is absent in stenohaline, osmoconforming species. CA expression is maintained at low, baseline levels in crabs acclimated to high salinity by the presence and action of this compound. The effects of the repressor appear to be reduced upon exposure to low salinity, allowing CA induction to occur.

Population Consumption of Fish and Invertebrate Prey by Striped Bass (Morone saxatilis) from Coastal Waters of Northern Massachusetts, USA

Seasonal, age-class, and population-level changes in diet and consumption demand of prey by striped bass residing in coastal waters of northern Massachusetts were investigated to determine their potential predatory impact on ecologically- and economically-important prey species. Most consumption by individual striped bass of ages 3–8 came from crustaceans and fish. More crusta ceans (50–78% of total consumption) than fish were consumed during June–July, while more fish (52–88% of total consumption) than crustaceans were consumed during August–September. Rock crabs Cancer irroratus and American lobsters Homarus americanus became more important to the production of striped bass as bass aged, but Atlantic menhaden Brevoortia tyrannus became less important. Together, the biomass of prey consumed by all age-classes in 2000 totaled over 5 575 t. Atlantic menhaden accounted for 29% of the total biomass consumed, followed by rock crabs (18%), American lobster (11%), and Atlantic herring Clupea harengus (3%). On a numerical basis, striped bass consumed seasonally over 3, 1 940, and 965 times the numbers of lobsters, rock crabs, and menhaden, respectively, taken annually by regional and statewide fisheries, suggesting striped bass may exert considerable predation pressure on these prey populations.

SUBSTRATE EFFECTS ON SURVIVAL, GROWTH AND DISPERSAL OF JUVENILE SEA SCALLOP, PLACOPECTEN MAGELLANICUS (GMELIN 1791)

Abstract In the scope of expanding seeding operations of juvenile scallops (Placopecten magellanicus) around Iles-de-la-Madeleine (Quebec, Canada), this study assesses the influence of substrate on growth, survival, dispersal and predation rates in controlled experiments. Three substrates were tested: gravel, sand and sand with empty scallop shells (to investigate the feasibility of modifying the substrate as a management strategy). Dispersal rates were studied with a substrate choice experiment. Predation experiments with two major scallop predators, the rock crab Cancer irroratus and the sea star Asterias vulgaris, were carried out on different substrate types. No significant difference in growth was observed among substrates and no mortality was observed during the experiment. This suggests other factors, such as hydrodynamics, may be more important than substrate. The dispersal rate was significantly greater on sandy substrate, whereas gravel and sand–shells had similar results. Crab predation was low...

Differential increase in AMPK and HSP70 mRNA expression with temperature in the rock crab, Cancer irroratus

5′ AMP activated protein kinase (AMPK) has been described in mammals as a “metabolic master switch” that regulates cellular [ATP]. Little is known about AMPK in invertebrates and the effect of temperature on AMPK expression. We tested the hypothesis that temperature stress increases AMPK mRNA levels in a decapod crustacean, and compared this increase to the temperature effect on HSP70 expression. Rock crabs, Cancer irroratus, were incubated for two days at 11, 24 or 27°C before sampling tissue from heart, hepatopancreas and claw muscle. RNA was extracted and reverse transcribed into cDNA. Quantitative PCR measured relative mRNA expression levels for AMPK and HSP70. Temperature had no effect on AMPK mRNA expression in hepatopancreas and heart but expression was 7.2 fold higher in the heart at all temperatures. AMPK expression in the claw muscle at 11°C was similar to the heart, but increased significantly 3.1 fold at 24°C. An insignificant 1.7 fold increase was observed at 27°C. HSP70 expression remained low in all three tissues at 11 and 24°C but increased significantly at 27°C (4.6, 6.6 and 14.5 fold, heart, hepatopancreas, claw muscle, respectively). Previous studies showed that high temperature leads to cellular ATP depletion. In the claw muscle increased AMPK expression at 24°C might prevent earlier ATP depletion even before the heat shock response starts in the form of HSP70 expression. Further experiments will elucidate the time course of these events in more detail. Supported by a New Investigator Award from MDIBL and a Research Career Enhancement Award from APS to M.F. and an INBRE fellowship to M.O.

Density Responses of Subarctic Coastal Marine Fish and Crabs to Artificial Reefs

AbstractStructurally complex habitats provide refuge from predators and higher invertebrate prey density—and therefore have higher densities of young fish—than do noncomplex (or barren) sea floors. Artificial reefs are used worldwide to increase seabed complexity in an attempt to increase fish and invertebrate densities and biomass. We employed small‐scale tetrahedral reefs to test three hypotheses: (1) that increased habitat complexity increases the density of individual demersal and epibenthic fish and crab species; (2) that association of affected species will be stronger during the day than at night; and (3) that the density of affected species will attenuate with distance from reefs. We deployed reefs with transects in Newman Sound, Newfoundland, Canada. Each reef transect was paired with a control transect and additional distant control transects along the 15‐m isobath. We estimated the densities of prevalent fish and crab species by means of scuba diving during the autumn of 1999 and summer of 2000. We observed a total of 722 fish and 362 crabs during this study. The densities of cunners Tautogolabrus adspersus in both years and winter flounder Pseudopleuronectes americanus and toad crabs Hyas araneus in 1999 significantly increased near artificial reefs during the day. Atlantic cod Gadus morhua and Greenland cod G. ogac showed some association with artificial reefs but not significantly so due to low daytime observations. For those species that showed an association with reefs, densities attenuated to baseline levels within 20 m from the reefs. None of the species examined were significantly associated with artificial reefs during the night. Yellowtail flounder Limanda ferruginea and rock crabs Cancer irroratus did not associate with reefs at any time.

Effect of the headline height of gillnets on species selectivity in the Gulf of Maine

Species selectivity of two types of low height experimental gillnets, 8 mesh deep (MD) and 12 MD, was compared with that of the standard cod gillnet (25 MD) and the tie-down flounder gillnet in inshore waters of the western Gulf of Maine. The experiment was designed to investigate whether using the lower nets could reduce the catch of the Atlantic cod ( Gadus morhua) while maintaining comparable catch of flounders. The results indicate that the 8 MD experimental net caught significantly less cod than the standard cod net ( t-test, P < 0.001), the tie-down flounder net ( t-test, P < 0.05), and the 12 MD experimental net ( P < 0.001). The size of cod caught by the tie-down nets was significantly smaller than those by the 8 MD or 12 MD nets ( P < 0.05). The 8 MD net caught marginally more flounders (combined number of yellowtail flounders, Limanda ferruginea, American plaice, Hippoglossoides platessoides, witch flounder, Glyptocephalus cynoglossus, and winter flounder, Pleuronectes americanus) than the standard cod net ( P = 0.05), but significantly less than the tie-down net ( P < 0.001). There are no significant differences in flounder catch rates between the 8 MD net and the 12 MD net ( P > 0.05). In addition, the tie-down net caught twice as much monkfish, Lophius americanus, sea raven, Hemitripterus americanus, and rock crab, Cancer irroratus, three times more American lobsters, Homarus americanus, and four to eight times more thorny skates, Amblyraja radiata, than the other three types of nets. On the other hand, the standard cod net caught significantly more spiny dogfish, Squalus acanthias, and white hake, Urophycis tenuis, than the other three nets ( P < 0.001). While further experiments are still needed in different fishing areas and seasons, the 8 MD low profile nets have great potential to substitute the standard cod net to maintain flounder catch rates while reducing the catch of cod.

Rock crab predation of juvenile sea scallops: the functional response and its implications for bottom culture

The functional response describes how consumption rate of individual predators changes as prey density changes, and can have important implications for the bottom culture of scallops. We examined (i) the functional response of rock crabs (Cancer irroratus) preying on juvenile sea scallops (Placopecten magellanicus); (ii) the effect of substrate type and scallop size on the functional response; and (iii) the underlying behavioural mechanisms of observed functional responses. Specifically, we quantified predation rate and behaviours, such as the proportion of time spent searching for prey, encounter rate between predators and prey and the outcomes of encounters, when individual rock crabs were offered a range of scallop density (2–50 or 11–111 scallops m−2) and two size classes of scallops (∼ ∼25 and ∼ ∼35 mm shell height) on two different substrate types (“glass-bottom” and “granule”). We found that crab predation rate on small juvenile scallops increased at a decelerating rate with prey density to a plateau at high prey density on both substrates, indicating a hyperbolic (type II) functional response. Crab predation rate on large juvenile scallops was independent of prey density (i.e., no functional response evident), suggesting that crabs were at their satiation level. Prey density did not influence any behaviour except encounter rate on small juvenile scallops, which increased as prey density increased. Substrate type influenced crab predation: maximum predation rate of crabs on small juvenile scallops and encounter rate with either size of juvenile scallops was lower on granule than on glass-bottom. Our results in the laboratory suggest that crabs could potentially be swamped if scallops are seeded at a high density in the field. However, many factors in the field may influence the functional response. For example, the presence of multiple prey types may lead to sigmoid functional responses, while the presence of many crab individuals may lead to aggregation of crabs in areas of high prey density.