We measured predation on bay anchovy Anchoa mitchilli eggs and larvae by abundant scyphomedusae Chrysaora quinquecirrha and ctenophores Mnemiopsis leidyi from gut contents, digestion rates, and densities of predators and prey during 9 d in July 1991 at 4 stations in Chesapeake Bay, USA. These predation rates were compared to egg and larval mortality rates measured concurrently in ichthyoplankton surveys. Daily predation by medusae and ctenophores was 19 * 13":) (mean + SD) of the eggs over the 20 h stage duration, with medusae responsible for 26 to 100% of the predation. These gelatinous predators accounted for 21 i 17 ":, of the total estimated daily egg-stage rnortality. On average, medusae consumed 29 + 14 O/u d -' of the larval bay anchovy, which was 41 35% of total estimated larval mortality. Predation on larvae by ctenophores was not detected. These predation effects are compared with those measured concurrently in free-drifting 3 2 m3 mesocosms. We conclude that medusae, which had high feeding rates but low abundances, and ctenophores, which had lour feeding rates but high abundances, were important predators of bay anchovy eggs and larvae in the mesohaline region of Chesapeake Bay

We conducted a predation experiment in 15.5 m3 in situ enclosures to estimate the predation potential of various juvenile and adult fishes on larval Atlantic herring Clupea harengus. Predators were herring, sprat Sprattus sprattus, and sandeels Hyperoplus lanceolatus and Amrnodytes tobianus. Herring consumed significantly fewer larvae in 24 h with less variability in the results than the other predators. Mean instantaneous daily mortality rate for herring was 0.001 (0.1 % d-l) whereas it was 0.392 (32 % d-l) for all the other predators. These values are 2.3 to 3.5 times those obtained in previous studies for jellyfish predators in enclosures. They are also greater than field estimates of natural mortahty for herring lanrae of comparable size, suggesting that encounters between herring larvae and larger fishes are rarer in the sea than in our experiment.

No abstract

ABSTRACT. The mesozooplankton taken in continuous plankton recorder samples from the central North Sea has changed from being numerically dominated by holoplanktonic calanoid copepod species from 1958 to the late 1970s to a situation where pluteus larvae of echinoid and ophiuro~d echlnoderms have been more abundant than any single holoplanktonic species in the 1980s and early 1990s. The abundance of the echinoderm larvae as a proportion of the zooplankton taken in the samples has followed a continuous increasing trend over the Dogger Bank, but off the eastern coast of northern England and southern Scotland the increase did not become obvlous until the 1980s. This trend is consistent with reported increases in abundance of the macrobenthos It is proposed that changes in the benthos have influenced the composition of the plankton.

Populations of Atlantic herring larvae Clupea harengus harengus L. were reared in 300 m3 in situ plastic enclosures for about 60 d