Sex-specific prey partitioning in breeding piscivorous birds examined via a novel, noninvasive approach

The helminth fauna of brown trout (Salmo trutta) in the Norwegian subalpine lake, Øvre Heimdalsvatn was studied by examination of gills, eyes, body cavity, kidney, stomach, pyloric region and intestine in a total of 112 brown trout randomly sampled in June, July, and September 2011. Ten helminth species, Discocotyle sagittata, Phyllodistomum umblae, Crepidostomum farionis, C. metoecus, Diplostomum sp., Proteocephalus sp., Cyathocephalus truncatus, Dibothriocephalus ditremus, D. dendriticus, and Capillaria sp. were identified. These data were compared to data from the period 1969 to 1972, just after the first record of the European minnow (Phoxinus phoxinus) in 1969. All ten helminth species, except D. dendriticus, were also present in 1969–72. However, a few major changes in infection intensities have occurred. The cestode D. ditremus and the trematode Diplostomum sp., both with piscivorous birds as final hosts, had markedly higher relative densities (abundance) in brown trout in 2011 compared to 1969–72, while the two Crepidostomum species showed a substantial decline in relative densities. We suggest that these changes may be indirectly related to the establishment and subsequent population increase of European minnow in the lake. The abundance of minnows may have increased the food basis for the piscivorous birds, primarily mergansers and the black-throated diver that now regularly forage in the lake. In addition, there have been changes in the littoral invertebrate community, including species serving as intermediate hosts of some of the brown trout parasites.

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Historic and contemporary use of catfish aquaculture by piscivorous birds in the Mississippi Delta

The Condor ◽

10.1093/condor/duaa036 ◽

2020 ◽

Author(s):

Paul C Burr ◽

Jimmy L Avery ◽

Garrett M Street ◽

Bronson K Strickland ◽

Brian S Dorr

Keyword(s):

Primary Source ◽

Annual Number ◽

Piscivorous Birds ◽

Ardea Herodias ◽

Efficient Management ◽

Cultured Fish ◽

Aquaculture Ponds ◽

Management Plans ◽

Aquaculture Area ◽

Aquaculture Facilities

Abstract Piscivorous birds are the primary source of catfish (Ictalurus spp.) depredation at aquaculture facilities in northwestern Mississippi. Of particular concern is the Double-crested Cormorant (Phalacrocorax auritus), which can cost aquaculture producers millions of dollars annually through the depredation of cultured fish. Historical research conducted in the early 2000s estimated cormorant use of aquaculture ponds in the region, but aquaculture area has decreased by more than 70% since those estimates were made. With less aquaculture available, we predicted cormorant densities on aquaculture would be greater today than historically. Applying a similar methodology as in historical studies, we used aerial surveys to collect data on cormorants at night roosts and using catfish aquaculture ponds during 3 consecutive winter seasons, beginning in 2015. Although the mean annual number of cormorants at roosts in the Delta during our study was 64% less than historically, we found no significant change in densities on aquaculture, suggesting that aquaculture area is likely the factor influencing cormorant occurrence in northwestern Mississippi. During contemporary surveys we also measured the abundance of Great Blue Herons (Ardea herodias) and Great Egrets (A. alba) on the aquaculture clusters, and built predictive models of abundance relative to variables associated with forage at and surrounding the clusters. We found abundance of all 3 species was strongly related to the amount of aquaculture area both within and surrounding a cluster, although patterns varied by species. Cormorant abundance was also greater on clusters with proportionately more food fish (≥20 cm in length) than fingerlings (<20 cm) and was positively related to the proximity and size of night roosts. The relationships described here can be used by producers and wildlife managers to predict the abundance of these piscivorous birds at aquaculture facilities and to design efficient management plans to mitigate potential impacts of depredation and disease.

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