Showing papers by "Jonathan H. Grabowski published in 2009"

Success of constructed oyster reefs in no-harvest sanctuaries: implications for restoration

Abstract: Dramatic declines in populations of the eastern oyster Crassostrea virginica are a symp- tom of degradation in many US Atlantic and Gulf of Mexico estuaries. We sampled 94 oyster reefs (88 constructed, 6 natural) within 11 no-harvest sanctuaries in estuaries of central and northern North Carolina, USA, to evaluate the success of oyster sanctuaries as a conservation tool. The sanctuaries have been in existence from 3 to 30 yr; 10 sanctuaries protect constructed ('restored') oyster reefs and 1 sanctuary protects natural reefs. Measurements of vertical relief, live oyster density, recruitment, abundance of market-sized oysters, and biomass as well as disease prevalence and severity indicated that 7 of the 11 sanctuaries met criteria for minimal success by having vertical relief >20 cm in height, living oysters (>10 oysters m -2 ), and evidence of recent recruitment in 1 of 2 yr of the survey. Most reefs within the 7 sanctuaries far surpassed these relatively low benchmarks. For reefs that failed, burial by sedimentation appeared to be the primary cause in 2 sanctuaries, poor water quality (low dissolved oxygen) in 1, and poor oyster recruitment in another. All intertidal reefs were successful and had significantly higher densities of all size categories of live oysters (spat, adult, marketable size) than subtidal oyster reefs. Disease prevalence and severity were low in sanctuary reefs despite high oyster densities and increased longevity of oysters on these reefs. Pronouncements that restora- tion of the native eastern oyster is a failure prove incorrect when a decade-long history of oyster reef sanctuaries is evaluated. The proposed introduction of a non-native oyster into the US Atlantic coast estuaries cannot be justified by claiming failure of native oyster restoration in light of promising suc- cesses within sanctuaries.

Evaluation of image-based multibeam sonar backscatter classification for benthic habitat discrimination and mapping at Stanton Banks, UK

Abstract: In recent years, efforts have increased to develop quantitative, computer-directed methods for segmentation of multibeam (MBES) backscatter data. This study utilises MBES backscatter data acquired at Stanton Banks (UK) and subsequently processed through the QTC-Multiview software environment in a bid to evaluate the program's ability to perform unsupervised classification. Statistical comparison with ground-truth data (grab, stills and video) enabled cross validation of acoustic segmentation and biological assemblages observed at the site. 132 unspecified variables were extracted from user-specified rectangular patches of the backscatter image, reduced to three vectors by PCA, then clustered and classified by the software. Multivariate analyses of ground-truth data were conducted on 75 stills images and 51 grab samples. Video footage coincident with the stills was divided into 30 s segments and coded by dominant substrate and species. Cross tabulation determined the interrelationship between software classifications, multivariate analysis of the biological assemblages and coded video segments. Multiview optimally identified 19 classes using the automated clustering engine. These were revised to 6 habitats a posteriori, using combined analysis of ground-truth data and Multiview data products. These habitats broadly correspond to major physiographic provinces within the region. Multivariate statistical analysis reveals low levels of assemblage similarity (< 35%) for samples occurring within Multiview classes, irrespective of the mode of acquisition. Coded video data is more spatially appropriate than the other methods of ground-truthing investigated, although it is less well suited to the extraction of truly quantitative data. Multivariate analysis indicates assemblages within physiographically distinct Multiview classes have a low degree of biological similarity, supporting the notion that abiotic proxies may be contraindicative of benthic assemblage variations. QTC-Multiview performs well as a mechanism for computer-assisted segmentation of MBES backscatter imagery into acoustic provinces; however a degree of caution is required prior to ascribing ecological significance to these classifications. (c) 2008 Elsevier Ltd. All rights reserved.

Resource dynamics influence the strength of non‐consumptive predator effects on prey

Abstract: Predators influence prey populations both by consuming individual prey, and by inducing changes in prey behaviour that limit reproduction and survival. Because prey trade-off predation risk for forageing gains, the magnitude of predators nonconsumptive effects should depend on resource availability. Studies of non-consumptive effects generally adopt either of two strategies: (i) maintaining a static ration of the preys resources; and (ii) using resource populations that vary dynamically in response to prey behaviour. Contrasting these experimental designs using meta-analysis, we evaluated whether resource dynamics influence the magnitude of non-consumptive effects on prey growth, survival, fecundity, population density, forageing rate and habitat use. Predators had a more negative effect on prey demography in dynamic- vs. static-resource experiments. Our results highlight the importance of resource dynamics in mediating the magnitude of non-consumptive effects of predators on prey, and illustrate the oftenunintended impacts of experimental design on estimates of effect size in ecological interactions.

The role of food limitation in lobster population dynamics in coastal Maine, United States, and New Brunswick, Canada

Abstract: Lobster (Homarus americanus) landings in Maine, United States have increased steadily over the past two decades to levels that are consistently more than triple the 40‐year (1950 to 1990) annual average of c. 9000 MT. Meanwhile, the use of herring (Clupea harengus) as bait has increased fourfold since the 1970s, and is currently subsidising lobster productivity in mid‐coast Maine by augmenting the diet and growth of large juvenile and young adult lobsters before they recruit to the fishery. We investigated whether herring bait in the eastern portions of the Gulf of Maine is also subsidising these lobster populations. In 2004, we compared stomach contents, tissue production via stable isotope ratios, and growth rates of lobsters from seasonally open (Cutler, Maine, United States) and closed (Dipper Harbour, New Brunswick, Canada) fishing areas in the spring, summer, and autumn. Contrary to previous results in mid‐coast Maine, lobsters from seasonally closed sites at Dipper Harbour outgrew those fr...