Bradley G. Stevens

Settlement, substratum preference, and survival of red king crab Paralithodes camtschaticus (Tilesius, 1815) glaucothoe on natural substrata in the laboratory

Abstract Preferences of red king crab (RKC) Paralithodes camtschaticus glaucothoes for a variety of live substrata were tested in the laboratory. Recently molted glaucothoes settled immediately on all substrata except a bare control tank. Preferred substrata (in decreasing order) were hydroids, algae, other (airstones, tank bottom), sand and worms, and did not change much until after molting to stage C1 at day 30. Survival to stage C1 was highest (93%) in the control tank, least (18%) on sand and intermediate (53–60%) on other substrata. Red king crab glaucothoes demonstrate similar preferences for structurally complex biological substrata in the laboratory as they do in the wild. Settlement on sand occurs only as a last resort and results in higher mortality. These results indicate the importance to settling larvae of biogenic oases that may be easily disturbed by hard-on-bottom fishing activities, and underscore the importance of conserving such habitats. Furthermore, knowledge of settlement, habitat selection and substratum preference are essential prior to considering the potential of king crabs for stock enhancement or aquaculture.

HATCH TIMING, INCUBATION PERIOD, AND REPRODUCTIVE CYCLE FOR CAPTIVE PRIMIPAROUS AND MULTIPAROUS RED KING CRAB, PARALITHODES CAMTSCHATICUS

Abstract Adolescent and mature female red king crab Paralithodes camtschaticus were captured by scuba divers and held in seawater tanks at the Kodiak Fisheries Research Center at ambient temperatures. Over three separate years, we monitored dates of embryo extrusion, timing and length of the hatching period, duration of embryological developmental, total degree-days, and volume and number of larvae released on a daily basis. Primiparous females extruded their embryos on a mean date of 3 February, two months prior to extrusion by multiparous crab. Mean hatching date for primiparous females occurred 16 days prior to that of multiparous females in 2001, and 12 days earlier in 2003. Primiparous females required a mean of 365 days and 2601 degree-days for complete embryonic development, whereas multiparous females required significantly less time, with a mean of 328 d, and 2482 degree-days. Although multiparous females were significantly larger than primiparous females, there was no significant difference between reproductive types in the time required for complete hatching (mean 32 days), total volume (mean 308 mL) or number of larvae released (mean 106, 884). Multiparous female crabs held in filtered or unfiltered (raw) seawater showed no differences in hatch timing, hatching days, or volume released. Virtually all larvae (91-95%) were released in the 4 hour period between 1800 and 2200 hours, and 50% were released in the first half-hour of darkness. Extended hatching exhibited by red king crabs is not synchronous with the occurrence of planktonic food sources, but may be an adaptation to the uncertainty of food availability. Shorter development times for multiparous embryos are necessary to complete the reproductive cycle prior to obligatory molting before extruding the next batch of eggs. This information was incorporated into a conceptual model of the first 3 years of reproduction for red king crab.

Timing and Duration of Larval Hatching for Blue King Crab Paralithodes Platypus Brandt, 1850 Held in the Laboratory

Abstract Female blue king crab, Paralithodes platypus, were captured from the Pribilof Islands, Alaska, in July and October 2003 and shipped to the Kodiak Fisheries Research Center. Crabs were held in chilled seawater at 2, 3.5, or 4°C until hatching, and final counts were 4, 6, and 10 crabs in each treatment, respectively. Mean size of all crabs was 123.9 mm CL and did not differ significantly between treatments. Individual crabs required an average of 29 days for complete hatching, regardless of holding temperature. Earliest hatching began on or before 20 February 2004. Crabs captured in July and held at 4°C hatched significantly earlier, with a mean hatch date of 13 March 2004, than crabs captured in October and held at 2°C or 3.5°C, both of which groups hatched on a mean date of 18 April 2004. Crabs released an average of 416 ml of larvae, and 110,033 first stage zoeae. Excluding two crabs for which complete volumes were not obtained, there were no differences in volume or number of larvae released by crabs held at different temperatures. The pattern, duration and timing of larval release exhibited by blue king crab is similar to that observed for other lithodid crabs including red king crab (P. camtschaticus), golden king crab (Lithodes aequispinus), and southern king crab (L. santolla). Extended hatching periods for king crabs may be an adaptation to environmental variability and the uncertainty of adequate food sources for larvae, and represent a reproductive strategy of diversified bet-hedging.

Thermal Effects on Embryonic Development and Hatching for Blue King Crab Paralithodes platypus (Brandt, 1850) Held in the Laboratory, and a Method for Predicting Dates of Hatching

Abstract Climate change may affect crab populations via thermal effects on embryo development and hatching. To test this, we measured the duration of development and hatching for the embryos of 11 blue king crabs Paralithodes platypus held at 2.3 ± 0.45, 4.3 ± 0.31, and 6.1 ± 0.61°C. Embryo area, length, and width, eye length and width, and percent yolk were measured biweekly from digital images, and hatching larvae were collected daily from individual crabs. Data were compared between eggs of identical age (weeks since fertilization). Temperature did not have a significant effect on embryo measurements, but did affect development indices (percent yolk and eye size). Hatching was significantly delayed at colder temperatures with about a 46-day difference from 2.3°C to 6.1°C. Length of development was related to temperature via a power function, and ranged from 410 ± 8 days at 6.1°C to 434 ± 11 days at 2.3°C. Length of hatching increased from 40 ± 4.6 days at 2.3°C to 55 ± 6.2 days at 6.1°C. A model for predicting hatching dates from an eye index was developed using a quadratic equation. Embryo development at 4.3 and 6.1°C was arrested between weeks 35 and 50; this evidence, plus other behavioral observations, suggests that crabs may be able to adjust development rates to partially compensate for temperature changes.

EMBRYO DEVELOPMENT AND MORPHOMETRY IN THE BLUE KING CRAB PARALITHODES PLATYPUS STUDIED BY USING IMAGE AND CLUSTER ANALYSIS

Abstract In this paper the embryonic development of laboratory-reared blue king crab, Paralithodes platypus, from the Pribilof Islands in the eastern Bering Sea is described. Developing embryos were removed from a female crab at various intervals, digitally photographed under a compound microscope and analyzed using Image-Pro Plus. Nine morphometric parameters were used, including seven measurements (total area, yolk area, embryo length and width, average diameter, eye length and width) and two calculated indices (percent yolk and elongation). First cell division was not apparent until day 4, after which divisions occurred daily until the blastopore appeared at day 28. A “V”-shaped embryo became apparent on day 114, followed by rapid appendage development. The eyes became pigmented by day 192. Hatching occurred from day 381 to day 409, and required at least 33 d to complete. Embryo area declined from an initial value of 0.95 mm2 on day 1–0.83 mm2 on day 72 and then increased to 1.28 mm2 on day 388. Growth of all characters reached a plateau between days 240 and 353, and then increased rapidly until the middle of hatching (day 390). Visual examination was better at defining early changes, but cluster analysis of morphometric measurements was a better technique for defining middle and later stages. Both techniques resulted in an optimum selection of 12 developmental stages. Embryonic development has been described for few decapod crustaceans, and no standard exists for defining developmental stages. Multivariate analysis of morphometric measurements may lead to improved understanding of crustacean embryogenesis, allow standardization of staging and enable studies of environmental influence on development. The technique also has applications in the aquaculture industry.

Effects of epizootic shell disease in American lobster Homarus americanus determined using a quantitative disease index

The incidence of epizootic shell disease in American lobster Homarus americanus has increased in southern New England, U.S.A., in the last decade, but few longitudinal studies have followed the disease progress in individual lobsters or demonstrated direct effects on mortality or growth. Diseased lobsters were held in the laboratory for 1 yr, and the progression of disease and its effects on molting, mortality, and growth were monitored. A quantitative disease index (QDI) was developed by measuring disease lesions in digital images of the carapace and expressing the result as a proportion of shell area. Some lobsters died due to high temperatures, but at least 13 of 55 lobsters (24%) died as a direct result of disease-related problems, mostly during molting, and there was a significant relationship between mortality and high values of the QDI. Lobsters that molted successfully were free of disease lesions, but many had exoskeletal deformities. There was no relationship between pre-molt size and disease severity, but molt increment was significantly correlated with premolt carapace length (CL) and negatively correlated with QDI. However, percentage growth was negatively correlated with QDI, but not with pre-molt CL. These significant lethal and sublethal effects of epizootic shell disease should be considered in lobster management.

Growth of Juvenile Red King Crabs, Paralithodes camtschaticus, Through Sequential Molts in the Laboratory

ABSTRACT Studies of crustacean growth, including king crabs, have typically used modal analysis rather than growth of individual crabs. I studied the individual growth of several hundred juvenile red king crabs (initial size 14–35 mm carapace length, CL) that were captured from a pod in Kodiak, Alaska, in July and August 2000, and retained in the laboratory for up to 14 months. During that period, 295 crabs molted at least once, including a subset of 52 that molted twice, and a subset of 9 that molted three times. Mean molt increment (MI) was 5.5 mm, and did not differ significantly between the three molts. MI increased significantly with premolt size for molt 1, and proportional growth rate at molt 1 was greater than that of molts 2 or 3. The first molt occurred earlier (August–October) and at warmer temperatures among smaller crabs than among larger crabs, which molted in January–February at colder temperatures. Post-molt CL for first molts was a simple ratio: Post-molt CL = 1.25 · (Pre-molt CL) (R2 = 0.9587, df = 291). Growth during the second molt was less than during the first molt, possibly reflecting a limitation imposed by the holding conditions. This is the first study to record growth information for individual juvenile red king crabs in the size range of 15–50 mm CL through sequential molts, and results can be used to fill a gap in current growth models.

Bait and the susceptibility of American lobsters Homarus americanus to epizootic shell disease

Shell disease (SD) has been observed in lobster populations for almost a hundred years, but recently, rates of an epizootic form of shell disease (ESD) have increased in the southern New England (USA) area. A large proportion of fish in the diet of American lobsters Homarus americanus has been linked to increased rates of SD. Therefore, the use of fish as lobster bait may be linked to increased ESD rates in lobsters. Lobsters from the western portion of Marthas Vineyard, MA (41 degrees N, 71 degrees W), were randomly divided into 3 groups of 16 and exposed to dietary treatments (100% herring; 48% crab, 48% blue mussel and 4% plant matter; or 50% herring, 24% crab, 24% mussel, 2% plant matter) to determine if lobster tissue delta15N levels reflected diet. The results of the feeding experiment confirmed that differences in diet are observed in the delta15N levels of lobster muscle tissue. The delta15N levels of tissue samples from 175 wild lobsters with varying degrees of ESD were unrelated to ESD severity but did indicate lobsters were eating large amounts of fish (bait). This result does not support the speculation that fish used as bait is contributing to ESD outbreaks in portions of the southern New England area.

Microbial analysis of a fish waste dump site in Alaska

The bacteriological impact of fish waste dumping in the ocean was evaluated by comparing heterotropic bacterial counts for seawater and sediment at the dump site with those from a control site. Prior to dumping, fish waste contained 3·0 × 107 bacteria g−1. Seawater near the ocean floor (bottom water) and sediment from the dump site contained 4·0 × 102 bacteria ml−1 and 2·2 × 106 bacteria g−1, respectively. Bottom water and sediment samples from the control site contained significantly lower bacterial counts (<102 bacteria g−1 and 6·1 × 104 bacteria g−1, respectively) than the dump site samples. For the five aerobic media tested, significant differences in bacterial counts occurred only for the fish waste samples. Anaerobic bacterial counts, for all samples except fish waste, were slightly less than the aerobic bacterial counts.

Age, size, and sexual maturity of channeled whelk (Busycotypus canaliculatus) in Buzzards Bay, Massachusetts

With the southern New England lobster fishery in distress, lobster fishermen have focused more effort toward harvesting channeled whelk (Busycotypus canaliculatus). However, minimal research has been conducted on the life history and growth rates of channeled whelk. Melongenid whelks generally grow slowly and mature late in life, a characteristic that can make them vulnerable to overfishing as fishing pressure increases. We sampled channeled whelk from Buzzards Bay, Massachusetts, in August 2010 and in July 2011, studied their gonad development by histology, and aged them by examining opercula. Males had a slower growth rate and a lower maximum size than females. Male whelk reached 50% maturity (SM50) at 115.5 mm shell length (SL) and at the age of 6.9 years. Female whelk reached SM50 at 155.3 mm SL and at the age of 8.6 years. With a minimum size limit of 69.9 mm (2.75 in) in shell width, males entered the fishery at 7.5 years, a few months after SM50, but females entered the fishery at 6.3 years, approximately 2 years before SM50. Increased fishing pressure combined with slow growth rates and the inability to reproduce before being harvested can easily constrain the long-term viability of the channeled whelk fishery in Massachusetts.