In order to determine whether the spatial distribution of sea nettles within mesocosms varied in direct response to dissolved oxygen treatments, we measured the position of sea nettles in six replicate mesocosms for each dissolved oxygen treatment.
We tested striped bass at higher dissolved oxygen concentrations than sea nettles because striped bass are less tolerant of low dissolved oxygen concentrations and tend to show at least partial avoidance at concentrations [is less than or equal to] 3 mg/L (Coutant 1985, Setzler-Hamilton and Hall 1991, Breitburg et al.
We conducted small-scale predation experiments to determine how low oxygen affects predation by sea nettles on prey with lower swimming speeds (copepods) or with no escape behavior (bay anchovy eggs).
Sea nettles, fish larvae, and zooplankton were acclimated to test dissolved oxygen concentrations in separate containers prior to the start of experiments.
Eggs were counted into batches of 100 and placed in the experimental containers just prior to the addition of sea nettles. As with small-scale larval predation experiments, egg-predation experiments were run as simple one-factor experiments with low and air-saturated dissolved oxygen treatments and predators in all containers; preliminary trials yielded near-100% recovery rates at both oxygen levels in the absence of predators (Table 3).
Additional comments on statistical analyses.--Variation in predator size was unavoidable because of day-to-day variation in the sizes of available sea nettles, and the growth of juvenile striped bass between early and late replicates.
Sea nettle predation on larvae.--The proportion of naked goby larvae captured by sea nettles in 1-[m.sup.3] mesocosms increased significantly at reduced dissolved oxygen concentrations from a low of 0.26 [+ or -] 0.03 in air-saturated controls to a high of 0.87 [+ or -] 0.02 in mesocosms maintained at 1.5 mg/L (Fig.
Furthermore, sea nettles did not significantly alter their position in mesocosms in response to dissolved oxygen concentrations (Table 5).
Consumption of naked goby larvae by sea nettles increased at low dissolved oxygen concentrations in small-scale experiments as it had in the 1-[m.sup.3] mesocosm experiments (Fig.
Low oxygen also led to decreased capture of copepods by sea nettles (Fig.
ANCOVA of effects of dissolved oxygen on predation on copepods by sea nettles in 80-L aquatubes.