Elevated summer temperature effects on megalopal and early juvenile ...

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Elevated summer temperature effects on megalopal and early juvenile development in the Dungeness crab, Cancer magister S.D. Sulkin, E. Mojica, and G.L. McKeen

Abstract: Commercial fishery stocks of the Dungeness crab, Cancer magister, are absent in regions of the middle Georgia Strait, Canada, where suitable benthic habitat occurs and where a plentiful supply of post-larval megalopae is present in nearby waters. This has led to speculation that high summertime temperatures in the region may result in lack of settlement or high juvenile mortality. The present study determined survival, time to metamorphosis, and growth of megalopae exposed to temperatures of 14, 18, and 228C. Newly metamorphosed juveniles were subjected to the same temperature treatments through the fourth crab instar. Although megalopal survival was not affected by temperature, juvenile crabs showed reduction in growth rate and high mortality in 228C. Results support the hypothesis that commercial crab fishery stocks in the region are absent because of high mortality of newly settled juveniles where summer water temperatures exceed 188C. Résumé : Il n’y a pas de stocks de crabes dormeurs, Cancer magister, d’importance commerciale dans la région moyenne du détroit de Georgia (Canada) où l’habitat benthique convient pourtant à cette espèce et où les eaux avoisinantes sont riches en mégalopes post-larvaires. Cet état de choses donne à penser que les températures élevées de l’été qu’on observe dans cette région pourraient expliquer pourquoi il ne s’établit pas de population ou pourraient être la cause d’une forte mortalité juvénile. Dans le cadre de l’étude présentée ici, on a déterminé la survie, l’époque de la métamorphose et la croissance de mégalopes exposées à des températures de 14, 18 et 228C. Des juvéniles récemment métamorphosés ont été soumis aux mêmes températures pendant le quatrième stade de leur développement. La température n’a pas eu d’influence sur la survie des mégalopes, mais on a constaté une baisse du taux de croissance et une forte mortalité chez les crabes juvéniles élevés à 228C. Les résultats de cette étude viennent étayer l’hypothèse selon laquelle l’absence de stocks d’importance commerciale dans la région étudiée est due à la forte mortalité des juvéniles qui viennent de s’implanter lorsque la température dépasse 188C en été. [Traduit par la Rédaction]

Introduction The Dungeness crab, Cancer magister, is fished commercially from central California to Unalaska Island, Alaska. Typical adult habitat includes the shallow sandy areas of the sublittoral zone along the outer coast, although a substantial population and fishery exists within the Puget Sound – Georgia Strait region traversing the Canada – United States border (Fig. 1). This “inland water” population exhibits characteristics of larval size and settlement patterns distinct from those of outer coast populations (Orensanz and Gallucci 1988; DeBrosse et al. 1990; Dinnel et al. 1993; Jamieson and Phillips 1993). Indeed, Jamieson and Phillips (1993) reported differences between Dungeness crab populations inhabiting the west (oceanic) and east (Georgia Strait) coasts of Vancouver Island and identified larval behavioral traits of the inland populations that may restrict their transport and isolate them from outer coast populations. In the Canadian portion of the inland water habitat, populations abundant enough to support a commercial fishery occur

Received August 16, 1995. Accepted March 11, 1996. J13039 S.D. Sulkin, E. Mojica, and G.L. McKeen. Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Road, Anacortes, WA 98221, U.S.A. Can. J. Fish. Aquat. Sci. 53: 2076–2079 (1996).

in the southern portion of the Strait of Georgia from the Fraser River estuary south to Boundary Bay (Fig. 1) (Jamieson 1985; Jamieson and Phillips 1993). However, such populations are rare along the east coast of Vancouver Island north of Nanaimo, B.C., even though extensive areas of suitable benthic habitat are present (Jamieson and Phillips 1993). Furthermore, it seems unlikely that the absence of fishery stocks in this middle Georgia Strait region is due to a lack of larvae available for settlement, since adjacent waters contain high densities of post-larval megalopae (Jamieson and Phillips, 1993). However, in the shallow benthic habitats typically occupied by early juvenile crabs (McMillan et al. 1995), water temperatures in the middle Georgia Strait may exceed 20oC during the late summer (Thomson 1981), the time of greatest occurrence of settlement of inland populations of megalopae (Orensanz and Gallucci 1988; Dinnel et al. 1993; Jamieson and Phillips 1993). Indeed, data from Baynes Sound (Fig. 1) indicated upper temperatures of 228C in the mid-intertidal waters in 1991–1992 (G. Jamieson, personal communication). By contrast, water temperatures in other regions of the inland water system known to support Dungeness settlement rarely exceed 168C during the summer (McMillan et al. 1995; personal observation). Sulkin and McKeen (1989) reported that zoeae raised in the laboratory could not develop successfully to the megalopa at 208C, although upper temperature limits of megalopae © 1996 NRC Canada

Sulkin et al. Fig. 1. Inland waters of the northern Puget Sound basin and Georgia Strait. Referenced locations: (A) Baynes Sound, (B) Nanaimo, B.C., (C) Vancouver, B.C., (D) Fraser River, (E) Boundary Bay, (F) Shannon Point Marine Center. Adapted from Jamieson and Phillips (1993, Fig. 3)

have not been reported. Jamieson and Phillips (1993) suggested that larvae in the middle Georgia Strait region may be stressed by exposure to temperature at or exceeding their upper tolerance limits. G. Jamieson (personal communication) further suggests that high shallow-water temperatures in the middle Georgia Strait region could account for the absence of commercial quantities of Dungeness crabs in otherwise suitable habitat. In the present study, we collected Dungeness crab megalopae from the northern Puget Sound basin and raised them at elevated temperatures to determine their ability to develop successfully to the first juvenile crab stage. We also determined the ability of newly metamorphosed crabs to survive, molt, and grow at the elevated summer temperatures present at otherwise suitable Dungeness crab habitat in the middle Georgia Strait region.

Methods Approximately 350 Cancer magister megalopae were captured at the water surface with a dipnet near the Shannon Point Marine Center, Anacortes, Wash. (Fig. 1). Megalopae were returned to the Marine Center where they were held in large trays fed with running seawater at ambient conditions (29 ppt salinity; 138C) that were identical to conditions at the nearby capture site. Megalopae were identifed tentatively to species based on size (DeBrosse et al. 1990) and all those used in experiments were confirmed as C. magister upon molt to the crab stage.

2077 Effect of temperature on megalopa development A total of 150 megalopae were selected randomly from the pool and placed in a glass bowl filled with filtered seawater at ambient conditions. Individual megalopae were then assigned randomly to styrene plastic culture boxes measuring 32 3 22 3 4 cm, each culture box containing 24 individual chambers each containing 50 mL of seawater and one megalopa. When all 150 megalopae had been distributed among the culture boxes, the boxes were distributed randomly among the three temperature treatments, each treatment receiving a total of 50 larvae. Investigations in our laboratory on the distribution of intermolt stages among cohorts of megalopae collected locally have indicated that cohort members are generally at the same stage of development when collected. Although it was not possible to identify specific intermolt stages of development of experimental subjects used in this study due to the destructive nature of the procedure (Hatfield 1983), we assumed that megalopae within a cohort collected together were of the same approximate stage of development and that random distribution of individuals among treatments would account for differences that might be present (Forward et al. 1994). The three temperature treatments used in the study were 14, 18, and 228C. Megalopae were exposed to higher temperatures (24–308C) in preliminary experiments; however, high mortality occurred in all cases within 24 h. The lowest temperature used (148C) is representative of temperatures in most of the inland water region during the late summer when the peak of megalopal settlement occurs (Dinnel et al. 1993; Jamieson and Phillips 1993); whereas 228C is representative of the highest temperatures reported for the middle Georgia Strait region. Megalopae were maintained in constant temperature incubators set to the appropriate temperature with a 14 h light : 10 h dark cycle. Megalopae were changed to clean, filtered seawater daily and fed freshly hatched nauplii of the brine shrimp, Artemia sp., in excess. Culture water was treated with the antibiotic chloramphenicol (5mg/L) (Sulkin and McKeen 1989). Each megalopa was examined daily to note incidence of mortality and molting. Megalopae that successfully underwent metamorphosis to the first juvenile crab instar (crab I) were maintained in laboratory culture through the molt to crab II at which time they were rinsed in freshwater, blotted dry, dried in a drying oven on a preweighed aluminum boat for 24 h, and weighed on a Mettler PE 160 analytical balance to the nearest 0.1 mg. Mean time to metamorphosis and mean dry weight of day 1 crab II individuals were compared among treatments. Effect of temperature on juvenile survival and growth Megalopae remaining in the original pool of those collected were maintained at 14oC until they metamorphosed to the crab I stage. Megalopae successfully molting to crab I were then randomly distributed among the three temperature treatments. Since not all megalopae molted to crab I on the same day, only day 1 crabs obtained from the mass culture in multiples exceeding three on any one day were used in the experiment. A total of 58 individuals was tested at each of the three temperature treatments. Crabs were maintained individually in the culture boxes following the same procedures as described above. Incidence of mortality and molting was noted daily to determine percent survival and instar duration for crab stages I–IV.

Results Megalopa development Although megalopal survival was not directly affected by the temperatures tested, both time to metamorphosis and growth differed among treatments. Survival to metamorphosis was virtually complete and consistent among all temperatures (Table 1). However, as temperature increased, mean time to © 1996 NRC Canada

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Can. J. Fish. Aquat. Sci. Vol. 53, 1996

Table 1. Percent metamorphosis and mean time to metamorphosis for megalopae raised in the three indicated temperatures. Temperature treatment (8C) 14 18 22

n

% metamorphosis

Time to metamorphosis (days)

50 50 50

98 100 98

7.5 (4.9)a 6.7 (4.4)a 3.3 (1.6)b

Note: Values for time to metamorphosis are means with SDs given in parentheses. Values with the same letter are not significantly different (P > 0.05) by nonparametric Tukey’s multiple comparison test. n, number of megalopae initially exposed to the temperature treatment.

metamorphosis decreased (Table 1). Because variances for megalopa duration were unequal (Bartlett’s test, P < 0.05; Zar 1984), the sample distributions were compared using the nonparametric Kruskal–Wallis ANOVA (Zar 1984), the results indicating significant differences among the samples (P < 0.001). Samples were further compared using the nonparametric Tukey’s multiple comparison test for unequal sample sizes (P = 0.05; Zar 1984). Times to metamorphosis at 14 and 188C were not significantly different, but both were significantly longer than time to metamorphosis at 228C (Table 1). Mean weight of day 1 crab II individuals also declined significantly with increasing temperature (Table 2; ANOVA, P < 0.05; Tukey’s test for pairwise comparisons of means, P = 0.05). Juvenile development Survival differed among temperatures during the early juvenile stages. A substantial reduction in survival occurred as early as crab II at 228C, with only 2% surviving the crab IV stage at this temperature (Table 3). This compares with 84 and 90% survival through crab IV at 148C and 188C, respectively. Duration of early juvenile crab instars varied with temperature. Duration of crab I was significantly longer at 148C than at the two higher temperatures (Table 4; Kruskal–Wallis ANOVA, P < 0.001; Tukey’s multiple comparison tests, P = 0.05). Durations of crab I instars were not significantly different from one another at 188C and 228C (Table 4). Significant differences in crab II and crab III instar durations were also found among temperature treatments (Table 4; Kruskal–Wallis ANOVA, Ps < 0.001). Results of Tukey’s tests (Ps = 0.05) for both instars indicated that instar duration at 188C was significantly shorter than at 14oC. However, instar duration at 228C was significantly longer than at 188C but not significantly different from that at 148C. Instar durations for crab IV were not compared among temperatures because high mortality at 22oC resulted in very small sample size.

Discussion Although Sulkin and McKeen (1989) reported that zoeae maintained throughout development at 208C could not develop to the megalopa, and Reed (1969) reported similar results at temperatures as high as 21.78C, the present study indicates that once the megalopa stage is reached, the postlarvae are tolerant of temperatures as high as 228C. At 228C,

Table 2. Mean weights, with SDs given in parentheses of day 1 crab II individuals in each of the indicated temperatures. Temperature treatment (8C)

Weight (mg)

14 18 22

9.9 (1.5)a 8.6 (1.7)b 7.7 (1.3)c

Note: Data were analyzed with a parametric one-way ANOVA and a subsequent Tukey pairwise comparison of means. Values with the same letter are not significantly different (P > 0.05).

Table 3. Percent survival through successive crab stages at indicated temperatures (n = 58 at beginning of experiment for each treatment). Temperature treatment (8C)

Crab stage I

II

III

IV

14 18 22

100 98 98

95 97 81

93 95 10

84 90 2

megalopae showed accelerated development to metamorphosis but smaller size at the crab II stage than was the case for megalopae maintained at lower temperatures. However, it is unlikely that megalopae advected into the middle Georgia Strait will be exposed to high surface temperatures through most of their planktonic period. Jamieson and Phillips (1993) reported that daytime distributions of megalopae were concentrated at depths exceeding 120 m, where the water temperatures were generally less than 108C. As megalopae entrained in middle Georgia Strait waters approach metamorphosis and move into more shallow regions prior to settlement, however, they are likely to encounter temperatures approaching 228C. Nevertheless, these results suggest that the relatively high summertime temperatures that megalopae may experience in the middle Georgia Strait shallow water benthic habitat should not prevent successful metamorphosis and settlement. Indeed, it might be argued that accelerated development will promote increased settlement success by reducing the amount of natural mortality due to such factors as predation while the megalopae are still planktonic prior to settlement. Results of the juvenile study suggest further that high temperatures that occur in the middle Georgia Strait shallowwater benthic environment should have little impact on the first juvenile crab instar, although some reduction in growth may occur. However, by the crab II stage, there is evidence of a significant reduction in growth rate at 228C and, by the crab III stage, this reduction in growth rate is accompanied by high mortality. The direct relationship between growth rate and temperature seen in crab I is expected and consistent with results from © 1996 NRC Canada

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Table 4. Mean duration (days), with SDs given in parentheses, of crab stages I–III for juvenile crabs cultured in the three indicated temperatures. Crab I Treatment temperature (8C) 14 18 22

Crab II

Crab III

n

Duration (days)

n

Duration (days)

n

Duration (days)

58 57 56

13.2 (1.9)a 10.5 (2.3)b 9.9 (1.6)b

55 56 47

14.4 (4.3)a 11.5 (2.9)b 15.1 (10.9)a

52 55 6

17.2 (4.4)a 15.3 (5.1)b 25.8 (16.5)a

Note: Within each stage, the three temperature treatments were compared by the Kruskall–Wallis ANOVA followed by the nonparametric Tukey’s multiple comparison test for unequal sample sizes. For each stage, values with the same letter are not significantly different. n, number of juvenile crabs that successfully molted to each crab stage in each temperature treatment.

field studies (e.g., McMillan et al. 1995). That this relationship breaks down as early as crab II, with instar duration becoming longer at 228C than at 188C, suggests the presence of temperature-induced stress at the higher temperature. This is supported by the substantial increase in mortality that occurs in the crab III stage at 228C. These results suggest that megalopal settlement should be a common phenomenon in suitable benthic habitat along the east coast of Vancouver Island north of Nanaimo, B.C., and that sampling in the region should reveal evidence of crab I and crab II instars. However, the high temperatures that typify the shallow-water benthic habitat during the period when peak settlement and early juvenile growth are occurring could result in high mortality of subsequent juvenile stages. The results are consistent with the hypothesis of Jamieson and Phillips (1993) that the absence of Dungeness crab fishery stocks in the middle Georgia Strait region is not due to failure of larval supply and settlement but to the effects of high temperature on subsequent juvenile survival.

Acknowledgements We thank Dr. Glen Jamieson, Pacific Biological Station, Nanaimo, B.C., for providing temperature data from Baynes Sound and for his helpful discussions about the topic and manuscript. E.M. was supported by a grant from the National Science Foundation establishing the Shannon Point Marine Center as a Research Experience for Undergraduates Site during the summer of 1993 (Grant No. OCE-9200195).

References DeBrosse, G., Sulkin, S., and Jamieson, G. 1990. Intraspecific morphological variability in megalopae of three sympatric species of the genus Cancer (Brachyura: Cancridae). J. Crustacean Biol. 10: 315–329.

Dinnel, P.A., Armstrong, D.A., and McMillan, R.O. 1993. Evidence for multiple recruitment-cohorts of Puget Sound Dungeness crab, Cancer magister. Mar. Biol. 115: 53–63. Forward, R., Jr., Frankel, D., and Rittschof, D. 1994. Molting of megalopae from the blue crab Callinectes sapidus: effects of offshore and estuarine cues. Mar. Ecol. Prog. Ser. 113: 55–59. Hatfield, S.E. 1983. Intermolt staging and distribution of Dungeness crab, Cancer magister, megalopae. In Life history, environment, and mariculture studies of Dungeness crab, Cancer magister, with emphasis on the central California fishery resource. Edited by P.W. Wild and R.N. Tasto. Calif. Dep. Fish Game Fish Bull. No. 172. pp. 85–96. Jamieson, G. 1985. The Dungeness crab, Cancer magister, fisheries of British Columbia. In Proceedings, Symposium on Dungeness Crab Biology and Management, 9–11 Oct. 1984, Anchorage, Alaska. Program Coordinator: B.R. Melteff. Lowell Wakefield Fish. Symp. Ser. Alaska Sea Grant Rep. No. 85–3. pp. 37–60. Jamieson, G., and Phillips, A. 1993. Megalopal spatial distribution and stock separation in Dungeness crab (Cancer magister). Can. J. Fish. Aquat. Sci. 50: 416–429. McMillan, R., Armstrong, D., and Dinnel, P. 1995. Comparison of intertidal habitat use and growth rates of two northern Puget Sound cohorts of 0+ age Dungeness crab, Cancer magister. Estuaries, 18: 390–398. Orensanz, J., and Gallucci, V. 1988. Comparative study of postlarval life history schedules in four sympatric species of Cancer (Decapoda:Brachyura:Cancridae). J. Crustacean Biol. 8 187–220. Reed, P. 1969. Culture methods and effects of temperature and salinity on survival and growth of Dungeness crab (Cancer magister) larvae in the laboratory. J. Fish. Res. Board. Can. 26: 389–397. Sulkin, S., and McKeen, G. 1989. Laboratory study of survival and duration of individual zoeal stages as a function of temperature in the brachyuran crab, Cancer magister. Mar. Biol.103: 31–37. Thomson, R.E. 1981. Oceanography of the British Columbia coast. Can. Spec. Publ. Fish. Aquat. Sci. 56. Zar, J.H. 1984. Biostatistical analysis. Prentice-Hall, Englewood Cliffs, N.J.

© 1996 NRC Canada