Reference

Abstract

Hatchery-reared striped bass Morone saxatilis have been stocked into the Patuxent River, Maryland, since 1985 as part of a multiagency striped bass restoration effort.  Fingerlings tagged with binary-coded wire tags (CWTs) are released as phase I fish (35-50 mm) in July and as phase II fish (150-200 mm) in November.  Mortality rates have been reported for hatchery fish in the Patuxent River; however, the initial loss of hatchery fish to predation is unknown.  This study attempts to (1) characterize initial predation on hatchery fish (species and size distribution of predators, and selectivity by predators for certain attributes of hatchery fingerlings) and (2) estimate initial loss of hatchery fish to predation.  The second objective (estimation of loss) is ongoing and therefore will not be reported here.

Phase II striped bass fingerlings were released into the Patuxent River on four consecutive days.  Fingerlings were marked daily prior to release with a uniquely coded CWT.  Each day, prior to release, a subsample was additionally marked with Floy tags (Floy Tag Co., Seattle, Washington).  Predators were sampled after peak feeding on each day of release.  Prior to predator sampling, an otter trawl was used to establish the ratio of wild to hatchery fish in the river.  Stomach contents were examined for number of wild versus hatchery striped bass, percent Floy-tagged hatchery fish, and size distribution of hatchery fish.

Striped bass was found to be the primary predator species at the sampling site (Patuxent River, river mile 24).  Other potential predator species (channel catfish Ictalurus punctatus, white catfish Ameiurus catus, and sporadically, Atlantic croaker Micropogonias undulates) were determined to be unimportant.  Of the predator striped bass captured with identifiable prey, the average number of hatchery fingerlings per predator stomach was 1.43.  Few cases of cannibalism appear in the literature; however, striped bass are known to be opportunistic feeders.  Cannibalism also has been shown to be a mechanism for density-dependent regulation of fish populations.

Length-frequency distribution of prerelease versus postcapture (in predator stomachs) hatchery striped bass (tagged with CWTs only) were compared and shown to be significantly different (95% confidence interval).  Predators appear to be selecting smaller hatchery fish.  Similarly, there was a significant difference between the percentage of Floy-tagged fish to hatchery fish without Floy tags in prerelease versus postcapture samples (5% versus 25%, respectively), indicating that predators may be selecting Floy-tagged fish.  Floy-tagged fish and small hatchery fish are possibly more vulnerable than other fingerlings, due to more awkward or slower swimming.  Ratios of wild to hatchery fish in trawl samples were not found to be significantly different from the ratio of wild to hatchery fish found in predator stomachs.

Tag

Coded Wire Tag (CWT)

Objective