No abstract

Approximately 255 humpback whales, Megaptera novaeangliae, moved through the waters off the Bay de Verde Peninsula, Newfoundland, between 1 June and 10 August 1978. Their distribution and northward migration speed were closely related to the density of depth sounder traces in the 80–120 m depth range, believed to have been made by postspawning schools of the humpbacks' major local prey, the capelin, Mallotus villosus. Residence times of individual humpbacks were usually less than 3 days. Only one animal had a preferred range within the study area.

Diurnal variations in pH and dissolved oxygen (DO) concentrations are common seasonal phenomena in many eutrophic estuaries, yet few studies have investigated the concurrent effects of low pH and low DO on marine organisms inhabiting these coastal systems. Here, we assess the effects of diurnal variations in pH and DO on the early-life history of two bivalve species native to Northeast US estuaries, the bay scallop (Argopecten irradians) and hard clam (Mercenaria mercenaria). In one set of experiments, larval-and juvenile-life stage bivalves were exposed to ambient conditions (pH T ∼ 7.9), two continuously-low pH levels (pH T ∼ 7.3 and 7.6), and diurnal fluctuations between the ambient and low conditions yielding mean pH levels equal to the intermediate pH levels. In a second set of experiments, larval bivalves were exposed to ambient conditions (pH T ∼ 7.9, DO ∼ 7 mg L −1 ), two levels of low pH and DO (pH T ∼ 7.2, DO ∼1 mg L −1 ; pH ∼ 7.4, DO ∼ 4 mg L −1 T ) and diurnal fluctuations of both pH and DO between the ambient and low pH/DO levels that resulted in mean pH and DO levels equal to the intermediate pH and DO levels. Diurnal acidification treatments with ambient DO levels yielded survival rates for both species at both life stages that were consistent with the survival of individuals exposed to the same mean level of chronic pH with juveniles being more resistant to acidification than larvae. In contrast, when both pH and DO varied diurnally, the survival rates of larval bivalves were significantly lower than the survival of individuals chronically exposed to the same mean levels of pH and DO, an indication that bivalves were physiologically more vulnerable to concurrent fluctuations of both parameters compared to acidification alone. While both species displayed sensitivities to diurnal fluctuations in pH and DO, scallops were relatively more susceptible than hard clams. Since many shallow eutrophic estuaries presently experience diurnal cycles of both pH and DO when early-life stages of bivalves are present in estuaries, the populations of the bivalves studied are likely impacted by these conditions which are likely to intensify with climate change.

We examined the planktonic ingress of fiddler crab megalopae into the Newport River Estuary, NC, using hog's hair collectors to determine whether there were differences in patterns of ingress among the 3 Uca species in this estuary. In 2 consecutive summers, we documented peaks in settlement of fiddler crab megalopae onto passive larval collectors at the quarter moons (neap tides), corresponding with longer durations of flood tides during darkness. However, molecular identifications revealed that over 80% of the megalopae settling on collectors were U. pugnax, even though there are millions of U. pugilator on sandflats within 1 km of the collection site and U. minax are common in the estuary. Plankton net samples taken on the same nights as collector settlement confirmed that megalopae of all 3 Uca species are common in the plankton. The proportions of species in the plankton and settling on collectors were significantly different. These differences may reflect species-specific differences in thigmotactic behavior, or may indicate differences in times of transport within a tide, since collectors reflect settlement at the end of rising tide while plankton nets sample the abundance at the time of maximum flood tide current. Thus, hog's hair collectors may not be an unbiased sampling method for fiddler crab megalopae.