Evaluating the Risk of Establishing a Self-Sustaining Population of Non-Native Oysters Through Large-Scale Aquaculture in Chesapeake Bay

A multistep invasive species methodology was used to evaluate the probability that the deployment of sterile non-native oysters (Crassostrea ariakensis, called Asian oyster) in field-based aquaculture could give rise to a reproductive population in Chesapeake Bay. Several pathways that could potentially lead to the unintentional release of reproductive C. ariakensis from aquaculture were identified. For the four quantifiable pathways, a probability was estimated for each step based on knowledge of ecological rates. A conservative estimate for the number of reproductive C. ariakensis that could arise over one spawning season at a hypothetical aquaculture site was estimated. A statistical expansion of this probability estimated that it is highly likely that the cultivation of putatively sterile C. ariakensis would initiate a reproductive population in Chesapeake Bay over a 10-year time span. The potential benefits of restoration actions involving non-native species in estuarine ecosystems must be weighed against potential ecological risks. A probabilistic approach can provide a useful method for summarizing the risk of an unintended introduction that may occur despite a reasonable and presumably “safe” approach for realizing benefits of a non-native species in aquaculture. The results presented here have important implications for future proposals involving the commercial use of non-native species in natural ecosystems.     

Unaided dispersal risk of Magallana gigas into and around the UK: combining particle tracking modelling and environmental suitability scoring

Marine non-indigenous species are a significant threat to marine ecosystems with prevention of introduction and early detection considered to be the only effective management strategy. Knowledge of the unaided pathway has received relatively little attention, despite being integral to the implementation of robust monitoring and surveillance. Here, particle tracking modelling is combined with spatial analysis of environmental suitability, to highlight UK coastal areas at risk of introduction and spread of Magallana gigas by the unaided pathway. ‘Introduction into UK’ scenarios were based on spawning from the continental coast, Republic of Ireland, Channel Islands and Isle of Man and ‘spread within UK’ scenarios were based on spawning from known UK wild populations and aquaculture sites. Artificial structures were included as spawning sites in an introduction scenario. The UK coast was scored, based on parameters influencing larval settlement, to reflect environmental suitability. Risk maps were produced to highlight areas of the UK coast at elevated risk of introduction and spread of M. gigas by the unaided pathway. This study highlights that introduction of M. gigas into UK waters via the unaided pathway is possible, with offshore structures increasing the potential geographical extent of introduction. Further, there is potential for substantial secondary spread from aquaculture sites and wild populations in the UK. The results of the study are considered in the context of national M. gigas management, whilst the approach is contextualised more broadly as a tool to further understanding of a little-known, yet significant pathway.

     

Aquaculture facilities drive the introduction and establishment of non-native Oreochromis niloticus populations in Neotropical streams

This study investigated whether aquaculture facilities drive the introduction and establishment of the non-native Nile tilapia (Orechromis niloticus) in Neotropical streams, Brazil. Samples were taken from nine streams with different aquaculture occupation intensities (no, moderate, and intense) using the percentage of occupation of micro-watersheds by aquaculture ponds as a proxy for propagule pressure. The presence of aquaculture facilities and the percentage of aquaculture occupation were good predictors of the catch frequency and of densities of tilapias in the adjacent natural environment. In the streams under intense propagule pressure, females prevailed and high densities of young individuals comparing to adults were recorded. It suggests that the species was reproductively successful in the natural environment. In the streams under moderate propagule pressure, males prevailed, which indicates the likely capture of individuals escaped from aquaculture facilities. In general, our results show the positive influence of propagule pressure on the introduction and establishment of Nile tilapia in natural ecosystems, showing that aquaculture expansion of O. niloticus poses a threat to the conservation of aquatic biodiversity.

     

The European sea bass: a key marine fish model in the wild and in aquaculture

The European sea bass (Dicentrarchus labrax L.) is a marine fish of key economic and cultural importance in Europe. It is now more an aquaculture than a fisheries species (>96% of the production in 2016), although modern rearing techniques date back only from the late 1980s. It also has high interest for evolutionary studies, as it is composed of two semispecies (Atlantic and Mediterranean lineages) that have come into secondary contact following the last glaciation. Based on quantitative genetics studies of most traits of interest over the past 10–15 years, selective breeding programs are now applied to this species, which is at the beginning of its domestication process. The availability of a good quality reference genome has accelerated the development of new genomic resources, including SNP arrays that will enable genomic selection to improve genetic gain. There is a need to improve feed efficiency, both for economic and environmental reasons, but this will require novel phenotyping approaches. Further developments will likely focus on the understanding of genotype‐by‐environment interactions, which will be important both for efficient breeding of farmed stocks and for improving knowledge of the evolution of natural populations. At the interface between both, the domestication process must be better understood to improve production and also to fully evaluate the possible impact of aquaculture escapees on wild populations. The latter is an important question for all large‐scale aquaculture productions.     

Genetic interactions between marine finfish species in European aquaculture and wild conspecifics

The principal species of marine aquaculture in Europe are Atlantic salmon (Salmo salar), sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus). For Atlantic salmon and sea bass, a substantial part of total genetic variation is partitioned at the geographical population level. In the case of sea bream, gene flow across the Azores/Mediterranean scale appears to be extensive and population structuring is not detected. For Atlantic salmon and sea bass, natural population structure is at risk from genetic interaction with escaped aquaculture conspecifics. The locally adaptive features of populations are at risk from interbreeding with non‐local aquaculture fish. Wild populations, generally, are at risk from interactions with aquaculture fish that have been subject to artificial selection or domestication. Atlantic salmon is the main European aquaculture species and its population genetics and ecology have been well‐studied. A general case regarding genetic interactions can be based on the information available for salmon and extended to cover other species, in the appropriate context. A generalized flow chart for interactions is presented. Salmon escape from aquaculture at all life stages, and some survive to breed among wild salmon. Reproductive fitness in the escaped fish is lower than in native, wild fish because of behavioural deficiencies at spawning. However, as the number of salmon in aquaculture greatly exceeds the number of wild fish, even small fractional rates of escape may result in the local presence of large numbers, and high frequencies, of escaped fish. At present, policy and legislation in relation to minimizing genetic interactions between wild and aquaculture fish is best developed for Atlantic salmon, through the recommendations of the Oslo Agreement developed by the North Atlantic Salmon Conservation Organization and subsequent agreements on their implementation. In future, the potential use of genetically modified fish in aquaculture will make additional policy development necessary. Improved containment is recommended as the key to minimizing the numbers and therefore the effects of escaped fish. Emergency recovery procedures are recommended as a back‐up measure in the case of containment failure. Reproductive sterility is recommended as a future key to eliminating the genetic potential of escaped fish. The maintenance of robust populations of wild fish is recommended as a key to minimizing the effects of escaped fish on wild populations.     

Climate change opens new frontiers for marine species in the Arctic: Current trends and future invasion risks

Climate change and increased anthropogenic activities are expected to elevate the potential of introducing nonindigenous species (NIS) into the Arctic. Yet, the knowledge base needed to identify gaps and priorities for NIS research and management is limited. Here, we reviewed primary introduction events to each ecoregion of the marine Arctic realm to identify temporal and spatial patterns, likely source regions of NIS, and the putative introduction pathways. We included 54 introduction events representing 34 unique NIS. The rate of NIS discovery ranged from zero to four species per year between 1960 and 2015. The Iceland Shelf had the greatest number of introduction events (n = 14), followed by the Barents Sea (n = 11), and the Norwegian Sea (n = 11). Sixteen of the 54 introduction records had no known origins. The majority of those with known source regions were attributed to the Northeast Atlantic and the Northwest Pacific, 19 and 14 records, respectively. Some introduction events were attributed to multiple possible pathways. For these introductions, vessels transferred the greatest number of aquatic NIS (39%) to the Arctic, followed by natural spread (30%) and aquaculture activities (25%). Similar trends were found for introductions attributed to a single pathway. The phyla Arthropoda and Ochrophyta had the highest number of recorded introduction events, with 19 and 12 records, respectively. Recommendations including vector management, horizon scanning, early detection, rapid response, and a pan‐Arctic biodiversity inventory are considered in this paper. Our study provides a comprehensive record of primary introductions of NIS for marine environments in the circumpolar Arctic and identifies knowledge gaps and opportunities for NIS research and management. Ecosystems worldwide will face dramatic changes in the coming decades due to global change. Our findings contribute to the knowledge base needed to address two aspects of global change—invasive species and climate change.     

The genetic characteristics of invasive Largemouth Bass in southern Brazil

Largemouth Bass (Micropterus salmoides) have been introduced on a global scale for sport fishing but represent a conservation concern given their documented negative impacts on native faunal diversity and abundance. Recent research using molecular data to characterize invasive Largemouth Bass populations elsewhere has demonstrated that populations are typically characterized by limited genetic diversity, or represent a combination of Largemouth Bass and Florida Bass (Micropterus floridanus). To test whether these traits were consistent with invasive populations in Brazil, we generated mitochondrial sequence data from four established populations of Largemouth Bass collected in southern Brazil as well as a local aquaculture facility to confirm species identity and quantify levels of genetic diversity. We identified the exclusive presence of Largemouth Bass in the region and observed limited levels of haplotype (haplotype diversity = 0.0684, SE = 0.038) and nucleotide diversity (0.0003, SE = 0.0002) which suggested the presence of a founder effect associated with introduction. Each of the four populations were dominated by a single haplotype that was identical to one recovered from a nearby aquaculture facility, which identified this facility as a potential introduction source.     

Application of 16s rDNA and cytochrome <Emphasis Type="Italic">b</Emphasis> ribosomal markers in studies of lineage and fish populations structure of aquatic species

The most economically important form of aquaculture is fish farming, which is an industry that accounts for an ever increasing share of world fishery production. Molecular markers can be used to enhance the productivity of the aquaculture and fish industries to meet the increasing demand. Molecular markers can be identified via a DNA test regardless of the developmental stage, age or environmental challenges experienced by the organism. The application of 16s and cytochrome b markers has enabled rapid progress in investigations of genetic variability and inbreeding, parentage assignments, species and strain identification and the construction of high resolution genetic linkage maps for aquaculture fisheries. In this review, the advantages of principles and potential power tools of 16s and cytochrome b markers are discussed. Main findings in term of trend, aspects and debates on the reviewed issue made from the model of aquatic species for the benefit of aquaculture genomics and aquaculture genetics research are discussed. The concepts in this review are illustrated with various research examples and results that relate theory to reality and provide a strong review of the current status of these biotechnology topics.     

Foliose Ulva Species Show Considerable Inter‐Specific Genetic Diversity,Low Intra‐Specific Genetic Variation,and the Rare Occurrence of Inter‐Specific Hybrids in the Wild

Foliose Ulva spp. have become increasingly important worldwide for their environmental and financial impacts. A large number of such Ulva species have rapid reproduction and proliferation habits, which explains why they are responsible for Ulva blooms, known as “green tides”, having dramatic negative effects on coastal ecosystems, but also making them attractive for aquaculture applications. Despite the increasing interest in the genus Ulva, particularly on the larger foliose species for aquaculture, their inter‐ and intra‐specific genetic diversity is still poorly described. We compared the cytoplasmic genome (chloroplast and mitochondrion) of 110 strains of large distromatic foliose Ulva from Ireland, Brittany (France), the Netherlands and Portugal. We found six different species, with high levels of inter‐specific genetic diversity, despite highly similar or overlapping morphologies. Genetic variation was as high as 82 SNPs/kb between Ulva pseudorotundata and U. laetevirens, indicating considerable genetic diversity. On the other hand, intra‐specific genetic diversity was relatively low, with only 36 variant sites (0.03 SNPs/kb) in the mitochondrial genome of the 29 Ulva rigida individuals found in this study, despite different geographical origins. The use of next‐generation sequencing allowed for the detection of a single inter‐species hybrid between two genetically closely related species, U. laetevirens, and U. rigida, among the 110 strains analyzed in this study. Altogether, this study represents an important advance in our understanding of Ulva biology and provides genetic information for genomic selection of large foliose strains in aquaculture.     

Disease in marine aquaculture

It has become almost a truism that success in intensive production of animals must be based in part on development of methods for disease diagnosis and control. Excellent progress has been made in methods of diagnosis for major pathogens of cultivated fish, crustacean and molluscan species. In many instances these have proved to be facultative pathogens, able to exert severe effects in populations of animals under other stresses (marginal physical or chemical conditions; overcrowding). The concept of stress management as a critical prophylactic measure is not new, but its significance is being demonstrated repeatedly. The particular relationship of water quality and facultative pathogens such asVibrio, Pseudomonas andAeromonas species has been especially apparent. Virus diseases of marine vertebrates and invertebrates — little known two decades ago — are now recognized to be of significance to aquaculture. Virus infections of oysters, clams, shrimps and crabs have been described, and mortalities have been attributed to them. Several virus diseases of fish have also been recognized as potential or actual problems in culture. In some instances, the pathogens seem to be latent in natural populations, and may be provoked into patency by stresses of artificial environments. One of the most promising approaches to disease prophylaxis is through immunization. Fish respond well to various vaccination procedures, and new non-stressing methods have been developed. Vibriosis — probably the most severe disease of ocean-reared salmon — has been controlled to a great extent through use of a polyvalent bacterin, which can be modified as new pathogenic strains are isolated. Prophylactic immunization for other bacterial diseases of cultivated fish has been attempted, especially in Japan, with some success. There is also some evidence that the larger crustaceans may be immunologically responsive, and that at least short-term protection may be afforded to cultured populations. Some progress has been made in marine disease control through chemical treatment in intensive culture systems, principally through application and modification of methods developed for freshwater aquaculture. Major constraints to use of chemicals are restrictions due to public health concerns about food contamination, and the negative effects of some chemicals on biological filters and on algal food production. There is a continuing need, however, for development of specific treatments for acute disease episodes — such as the nitrofurans, developed in Japan, which are effective for some bacterial diseases. The history of aquaculture — freshwater as well as marine — has been characterized by transfers and introductions of species to waters beyond their present ranges. The process continues, and carries with it the possibility of transfers of pathogens to native species and to the recipient culture environments. International groups are attempting to define codes of practice to govern such mass movements, but examples of introductions of real or potential pathogens already exist. The most recent and the most dramatic is the world wide transfer of a virus pathogen of penaeid shrimps. Earlier examples include the introduction of a protozoan pathogen of salmonids to the western hemisphere, and the introduction of a parasitic copepod from the Far East to the west coast of North America and to France. The conclusion is inevitable — diseases are substantial deterrents to aquaculture production. Diagnostic and control procedures are and will be important components of emerging aquaculture technology.     

Status and impact of rural aquaculture practice on Lake Victoria basin wetlands

Rural aquaculture in Lake Victoria basin is a fast increasing nontraditional farming activity which if not appropriately practised will lead to degradation of the wetlands. As part of a study to develop appropriate guidelines and model systems for wetlands‐based rural aquaculture in the basin a survey was conducted to assess the status and the ecological and socioeconomic impact of rural aquaculture on wetlands and wetlands communities. Aquaculture practice was found to be common but not as a major activity. Aquaculture in the wetlands can be described as a low input‐low output production activity and subsistence based on ponds under 400 m² using free seed from public agencies with hardly any supplementary feeding. Men owned most of the ponds and women only contributed to the management of the fishponds by feeding the fish. Poorly constructed ponds and loss during harvesting have led to the escape of cultured species into the wild. Introduction of nonnative species in the basin has already led to wide ranging ecological, environmental and socioeconomic changes whose impact and usefulness are still very much contentious. Repeat of such scenarios can be avoided if appropriate and science‐based models for rural aquaculture farming are developed, tested and disseminated to the communities.     

DNA分子标记技术及其在水产动物遗传上的应用研究

随着DNA分子标记技术的发展,其在动物遗传上发挥了重大作用,使用DNA分子标记可以观察到整个基因组的遗传多样性。目前,在水产养殖种类中使用的遗传标记主要包括线粒体DNA、RFLP、RAPD、AFLP、微卫星、SNP和EST标记。DNA分子标记的应用使得人们对水产养殖动物的遗传多样性、近亲繁殖、种类和品系鉴定以及遗传连锁图谱建立的研究都取得了很大进展,也加快了数量性状位点(QTL)基因的鉴定作为分子标记辅助选择(MAS)的研究。将这些标记技术在水产动物上的应用进行了论述,以及如何从人类基因组工程和斑马鱼这种模式鱼的研究中得到启发,更好的应用于水产动物基因组学和遗传学研究做一讨论。     

FIRST RECORD OF GRACILARIA VERMICULOPHYLLA (GRACILARIALES,RHODOPHYTA) IN THE PO DELTA LAGOONS,MEDITERRANEAN SEA (ITALY)1

Molecular outcomes led us to report the first occurrence of the invasive alien species Gracilaria vermiculophylla (Ohmi) Papenf. in the Mediterranean Sea. This species was recorded for the first time in the Po Delta lagoons in May and October 2008, probably introduced by the importation of the Manila clam Tapes philippinarum. At present, G. vermiculophylla is spread only near some clam‐farming areas, but its diffusion is expected to increase with the colonization of other lagoons where aquaculture is spread, as already observed for other alien species such as Agardhiella subulata and Solieria filiformis. The present study supplies further information on the morphology of this species, the ecological characteristics of the colonized areas, and the most probable introduction vector, confirming that the species spreading occurs in eutrophic and turbid coastal systems.     

Spatio-temporal variation of the zooplankton community in a tropical caldera lake with intensive aquaculture (Lake Taal,Philippines)

Zooplankton were collected from Lake Taal between January and December 2008 in order to test for differences in species composition and abundance between a lake basin with intensive fish cage (FC) aquaculture and an open water area without FCs. Most species showed similar patterns of occurrence in both basins while having differences in abundance. Several rotifer species were more abundant in FC sites most of the year, while for microcrustaceans higher abundances in FC sites only happened during the first 2 months. Their distribution is attributed to the presence of higher nutrient levels in FC sites as well as wind-induced basin-wide water movements during the different monsoon seasons which disperse plankton and nutrients from FC sites to other parts of the lake. Zooplanktonic indicators, such as the Brachionus–Trichocerca quotient (Q _B/T) and the ratio of calanoids to cladocerans and cyclopoids, clearly demonstrate the eutrophic state of the lake. A comparison with previous studies suggests that the lake was already eutrophic prior to the introduction of aquaculture in the 1970s. The trophic conditions in Lake Taal may be attributed to the lakes’ tropical and volcanic nature, with productivity further enhanced by increased nutrient input from aquaculture.     

Uncovering QTL for resistance and survival time to Philasterides dicentrarchi in turbot (Scophthalmus maximus)

Disease resistance‐related traits have received increasing importance in aquaculture breeding programs worldwide. Currently, genomic information offers new possibilities in breeding to address the improvement of this kind of traits. The turbot is one of the most promising European aquaculture species, and Philasterides dicentrarchi is a scuticociliate parasite causing fatal disease in farmed turbot. An appealing approach to fight against disease is to achieve a more robust broodstock, which could prevent or diminish the devastating effects of scuticociliatosis on farmed individuals. In the present study, a genome scan for quantitative trait loci (QTL) affecting resistance and survival time to P. dicentrarchi in four turbot families was carried out. The objectives were to identify QTL using different statistical approaches [linear regression (LR) and maximum likelihood (ML)] and to locate significantly associated markers for their application in genetic breeding strategies. Several genomic regions controlling resistance and survival time to P. dicentrarchi were detected. When analyzing each family separately, significant QTL for resistance were identified by the LR method in two linkage groups (LG1 and LG9) and for survival time in LG1, while the ML methodology identified QTL for resistance in LG9 and LG23 and for survival time in LG6 and LG23. The analysis of the total data set identified an additional significant QTL for resistance and survival time in LG3 with the LR method. Significant association between disease resistance‐related traits and genotypes was detected for several markers, a single one explaining up to 22% of the phenotypic variance. Obtained results will be essential to identify candidate genes for resistance and to apply them in marker‐assisted selection programs to improve turbot production.     

152 Non‐Indigenous and Invasive Marine Algae in Hawaii

The introduction of non‐indigenous species is currently viewed as one of the largest threats to global biodiversity. Non‐indigenous marine algae (NIMA) in temperate seas around the world have caused significant losses to ecosystem structure and function. However, the introduction of invasive species in tropical marine ecosystems has not typically been viewed as a significant threat despite the number of species that have been transported to reef regions around the world for open‐reef aquaculture. The research presented here represents some of the first quantified evidence of significant negative impacts of NIMA in tropical waters. This study characterized several ecological and physiological aspects of one the most successful and potentially threatening NIMA on Hawaii's coral reefs, K. alvarezii. Results of large‐scale surveys and a number of permanently established invader removal plots suggest that K. alvarezii is having negative impacts on native species diversity. Interactions between K. alvarezii and coral abundance were examined using photoquadrats and results indicate that the invader is causing coral death as a result of overgrowth and shading. Possible mechanisms influencing invader success including responses to nutrient enrichment, grazer consumption rates and reproductive characteristics were examined. Results from a number of experiments suggest that without mitigation K. alvarezii will continue to spread. In an effort to minimize negative impacts and prevent spread several management strategies were examined including manual removal, use of chemical and temperature treatments and enhancement of native sea urchins. While some of these tools are promising control options, rapid implementation is needed to prevent further damage.     

Contrasting patterns of genome‐wide polymorphism in the native and invasive range of the marine mollusc Crepidula fornicata

Selection processes are believed to be an important evolutionary driver behind the successful establishment of nonindigenous species, for instance through adaptation for invasiveness (e.g. dispersal mechanisms and reproductive allocation). However, evidence supporting this assumption is still scarce. Genome scans have often identified loci with atypical patterns of genetic differentiation (i.e. outliers) indicative of selection processes. Using microsatellite‐ and AFLP‐based genome scans, we looked for evidence of selection following the introduction of the mollusc Crepidula fornicata. Native to the northwestern Atlantic, this gastropod has become an emblematic invader since its introduction during the 19th and 20th centuries in the northeastern Atlantic and northeastern Pacific. We examined 683 individuals from seven native and 15 introduced populations spanning the latitudinal introduction and native ranges of the species. Our results confirmed the previously documented high genetic diversity in native and introduced populations with little genetic structure between the two ranges, a pattern typical of marine invaders. Analysing 344 loci, no outliers were detected between the introduced and native populations or in the introduced range. The genomic sampling may have been insufficient to reveal selection especially if it acts on traits determined by a few genes. Eight outliers were, however, identified within the native range, underlining a genetic singularity congruent with a well‐known biogeographical break along the Florida. Our results call into question the relevance of AFLP genome scans in detecting adaptation on the timescale of biological invasions: genome scans often reveal long‐term adaptation involving numerous genes throughout the genome but seem less effective in detecting recent adaptation from pre‐existing variation on polygenic traits. This study advocates other methods to detect selection effects during biological invasions—for example on phenotypic traits, although genome scans may remain useful for elucidating introduction histories.     

Aquaculture-induced changes to dynamics of a migratory host and specialist parasite: a case study of pink salmon and sea lice

Exchange of diseases between domesticated and wild animals is a rising concern for conservation. In the ocean, many species display life histories that separate juveniles from adults. For pink salmon (Oncorhynchus gorbuscha) and parasitic sea lice (Lepeophtheirus salmonis), infection of juvenile salmon in early marine life occurs near salmon sea-cage aquaculture sites and is associated with declining abundance of wild salmon. Here, we develop a theoretical model for the pink salmon/sea lice host–parasite system and use it to explore the effects of aquaculture hosts, acting as reservoirs, on dynamics. Because pink salmon have a 2-year lifespan, even- and odd-year lineages breed in alternate years in a given river. These lineages can have consistently different relative abundances, a phenomenon termed “line dominance”. These dominance relationships between host lineages serve as a useful probe for the dynamical effects of introducing aquaculture hosts into this host–parasite system. We demonstrate how parasite spillover (farm-to-wild transfer) and spillback (wild-to-farm transfer) with aquaculture hosts can either increase or decrease the line dominance in an affected wild population. The direction of the effect depends on the response of farms to wild-origin infection. If aquaculture parasites are managed to a constant abundance, independent of the intensity of infections from wild to farm, then line dominance increases. On the other hand, if wild-origin parasites on aquaculture hosts are proportionally controlled to their abundance then line dominance decreases.     

Piscivorous Bird Use of Aquaculture and Natural Water Bodies in Mississippi

Double-crested cormorants (Phalacrocorax auritus) and great egrets (Ardea alba) have an extensive history of human-wildlife conflict with the aquaculture industry of western Mississippi, USA, due to their depredation of cultured catfish (Ictalurus spp.). Although aquaculture is abundant, western Mississippi also contains naturally occurring water bodies that offer alternative forage opportunities to these species. How cormorants or egrets distribute themselves among these 2 foraging options is unknown, but it has been generally assumed each species uses aquaculture disproportionately more because of the high density of available prey. To test this assumption, we surveyed these species on aquaculture and naturally occurring water bodies using aerial surveys from October through April of 2015–2016, 2016–2017, and 2017–2018. We modeled the proportion of each species on aquaculture as a function of year, date, and weather-related variables using quasi-binomial generalized linear models. Egrets used aquaculture consistently more than what was proportionally available to them and use was not influenced by any of the variables we measured. Proportional use of aquaculture by cormorants was lowest during October through January but steadily increased through April, indicating a distribution shift toward aquaculture in the months immediately prior to their migration. The highest proportional use of aquaculture by cormorants occurred in 2016, a year when lethal control measures were not allowed against cormorants. Conversely, the least proportion of cormorants on aquaculture was in 2015 when cormorants could be lethally controlled under authority of an Aquaculture Depredation Order. This trend highlights the potential influence of changes in mortality risk, caused by changes in policy regarding lethal take of cormorants, on cormorant distribution between foraging options. © 2020 The Wildlife Society.