Prey depletion, especially in the high-oxygen treatment, may have led to an underestimate of the magnitude of oxygen effects on sea nettle consumption of fish eggs.
Moderately low dissolved oxygen concentrations are likely to affect striped bass:larvae encounter rates more strongly than sea nettle:larvae encounter rates.
Sea nettle predation decreased at low dissolved oxygen concentrations when prey had no escape response at any dissolved oxygen concentration (fish eggs), was little affected for the slower swimming of the two active prey tested (copepods), and, as in mesocosm experiments discussed earlier, increased for fish larvae that can frequently escape or avoid capture at high oxygen concentrations.
Sea nettle predation is likely to be important at both low dissolved oxygen concentrations tested for all prey used in mesocosm and small-scale experiments.
For example, complementary responses in these experiments by fish and sea nettle predators of fish larvae, or for predation by the sea nettle on two different early life stages of fish, could dampen the potential impact of low oxygen on secondary production in this system.
Naked goby, bay anchovy, sea nettles, and Acartia tonsa co-occur at salinities of [is less than] 5 (practical salinity scale) to nearly full-strength sea water (Lippson 1973, Fritzsche 1978, Jones 1978), and are therefore found together throughout the Chesapeake Bay system.
Larval bay anchovy (Anchoa mitchilli) were used as additional prey in some experiments (see the section on sea nettles in Mesocosm experiments).
Sea nettles used as predators in mesocosm and small-scale experiments were collected from the Patuxent River with a fine-mesh hand dip net either the day of the experiment or the day before, and were maintained in coarsely filtered (~224-[micro]m mesh) sea water.
General methods for mesocosm experiments.--Experiments were conducted in 1-[m.sup.3] mesocosms to test the effect of dissolved oxygen on predation of fish larvae by sea nettles and juvenile striped bass, and to determine if the interaction between dissolved oxygen and prey density affects predation by juvenile striped bass.
Field-collected zooplankton were added to tanks used for experiments testing direct effects of dissolved oxygen on predation by sea nettles and striped bass in order to help promote natural behavior of larvae and sea nettles.
Predators (sea nettles or juvenile striped bass) were acclimated in separate 20-L buckets, and added to mesocosms ~30 min after larvae.
Effects of dissolved oxygen on predation by sea nettles.--Sea nettle experiments in mesocosms were set up as randomized complete blocks (by date of replicate) with paired predator and no-predator controls at each dissolved oxygen concentration.