Abundance and migratory patterns of anadromous fish spawning runs in the Roanoke River, North Carolina
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2003-07-17
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Abstract
Anadromous fishes often support significant commercial and recreational fisheries. Obtaining the information needed for their effective management can be problematic, however, because of the difficulties associated with sampling coastal rivers. Objectives of this research were to: (1) evaluate two novel methods for characterizing the spawning runs of anadromous fishes in southeastern coastal rivers, and (2) develop a method for estimating the total size of a spawning run.
We operated a fishwheel on the Roanoke River, North Carolina during the spring seasons of 2000, 2001, and 2002, and conducted stationary, split-beam, side-looking hydroacoustic monitoring at the same site in 2001. The fishwheel captured 39 species over the three seasons, but fishwheel effectiveness appeared to vary among species and with changes in river conditions. The turning speed of the baskets was nearly always lower than what is considered optimal, suggesting that water velocities were too low for most effective operation. Daily fishwheel catches provided good information about the timing of migration for anadromous species that were captured in sufficient number, primarily striped bass and blueback herring. Catches of anadromous species varied substantially through each season, both due to the timing of migration and to changes in the fishwheel's effectiveness. Daily fishwheel catches provided information about the spawning runs at finer temporal scales than is typically possible, and the size distributions and annual catch rates of striped bass obtained by fishwheel sampling were similar to results from electrofishing.
We developed a model for estimating run size using data from split-beam sidelooking hydroacoustic monitoring. The model is based on counts of upstream migrants and makes adjustments for: (1) the proportion of the day that was sampled (temporal sampling fraction), (2) the proportion of the river that was sampled (spatial sampling fraction), (3) the proportion of fish detected that were included in the counts (detectability), and (4) the proportion of the counts comprised by the species of interest (species proportion). Future refinements of the model should include: (1) methods to account for variation in detection probability as it relates to a target's location in the beam, and (2) stratification procedures to use in calculating the spatial sampling fraction(s).
Stationary, split-beam, side-looking hydroacoustic monitoring was effective for assessing the abundance and temporal and spatial patterns in the 2001 spawning run of striped bass. The striped bass run was characterized by a number of distinct peaks, which were not related to river discharge. The pattern of the run was generally consistent with that obtained from daily fishwheel catches. We found that the number of striped bass migrating upstream was significantly greater during nighttime hours than during the day. Most striped bass migrated within 1.5 m of the river bottom and were evenly distributed across the channel, as would be expected based on river morphology and flow conditions. By combining acoustic counts of upstream migrants with supplemental sampling data from the fishwheel, we estimated that 262,789 striped bass participated in the spawning migration in 2001. Our estimate is in good agreement with run size estimates for 1956-1980, but is considerably lower than more recent estimates based on virtual population analysis modeling.
Fishwheels and split-beam, side-looking hydroacoustics both hold promise as sampling methods for anadromous fishes in southeastern coastal rivers. Each gear provides valuable information about spawning runs that is difficult to obtain by more traditional means. Reliable run size estimates and detailed information about the temporal and spatial patterns in spawning runs should provide managers with a stronger foundation for building sustainable fisheries.
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run size, spawning runs, anadromous fishes, relative abundance, migratory patterns, Roanoke River, fishwheel, hydroacoustics, alewife, American shad, hickory shad, blueback herring, <i>Morone saxatilis</i>, striped bass
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Degree
MS
Discipline
Fisheries and Wildlife Sciences
