Dolphin Stocks Research Articles

Seasonal Prevalence of Skin Lesions on Dolphins across a Natural Salinity Gradient

Bottlenose dolphins (Tursiops truncatus) inhabit waters across a broad natural salinity gradient and exhibit changes in skin condition based on the quality of their environment. Prolonged exposure to low salinities (≤10–20 ppt) degenerates the epidermal barrier and causes cutaneous lesions in dolphins, while the role of high salinity exposure (>35 ppt) in lesion development remains unknown. We assessed seasonal lesion prevalence in three free-ranging dolphin stocks inhabiting coastal Gulf of Mexico (GoM) waters of different salinities (0–30 ppt, 22–35 ppt, and 36+ ppt) using images of dolphin bodies. Lesions were documented on 44% of the dolphins photographed (n = 432), and lesion occurrence was significantly related to cold seasons and water temperatures but not salinity. Cold water temperatures may heighten dolphin susceptibility to infectious pathogens and disease and compound the effects of anthropogenic pollutants in the GoM. As dolphins are a bioindicator species of marine habitat welfare, natural studies assessing dolphin skin may reveal environmental degradation with potential impacts on marine ecosystems and human health.

Applying the phylogeographic concept to identify franciscana dolphin stocks: implications to meet management objectives

High numbers of franciscanas (Pontoporia blainvillei) have been bycaught in gillnets for at least four decades. The impact is strong but not homogeneous along the species distribution range, and there is evidence that at least one local population is declining. Reliable stock determination is important to evaluate how bycatch affects stocks so that they can be preserved through local management actions. Following the phylogeographic approach of Dizon et al. (1992), which applies a hierarchical classification scheme to stock designations, available information relevant for franciscana stock discreteness was reviewed, including data on distribution, population response, phenotype and genotype. Data on mtDNA, morphometrics and population parameters all together provide evidence for splitting the species into four management stocks: two inhabiting coastal waters of Brazil; the third occurring in Rio Grande do Sul State (southern Brazil) and Uruguay, and the fourth inhabiting coastal Argentine waters. The areas where these stocks occur are herein called Franciscana Management Areas or FMA I to IV. Although in some cases the evidence for such sub-division into four stocks is weak due to lack of data, the use of the proposed classification into four FMAs is recommended to warrant effective management on a local scale.

Review of potential line-transect methodologies for estimating abundance of dolphin stocks in the eastern tropical Pacific

A twelve-year hiatus in fishery-independent marine mammal surveys in the eastern tropical Pacific Ocean (ETP), combined with a mandate to monitor dolphin stock status under international agreements and the need for reliable stock status information to set dolphin bycatch limits in the tuna purse-seine fishery, has renewed debate about how best to assess and monitor ETP dolphin stock status. The high cost of replicating previous ship-based surveys has intensified this debate. In this review, transect methods for estimating animal abundance from dedicated research surveys are considered, with a focus on both contemporary and potential methods suitable for surveying large areas for dolphin species that can form large, multi-species aggregations. Covered in this review are potential improvements to the previous ship-based survey methodology, other ship-based methods, alternative approaches based on high-resolution imagery and passive acoustics, and combinations of ship-based and alternative approaches. It is concluded that for immediate management needs, ship-based surveys, with some suggested modifications to improve precision, are the only reliable option despite their high cost. However, it is recommended that a top research priority should be development of composite methods. Pilot studies on the use of high-resolution imagery and passive acoustics for development of indices of relative abundance to be used in composite methods should be part of any future ship-based survey efforts.

Mark-recapture estimates suggest declines in abundance of common bottlenose dolphin stocks in the main Hawaiian Islands

Species conservation relies on understanding population demographics, yet this information is lacking for many species and populations. Four stocks of common bottlenose dolphinsTursiops truncatusinhabiting the waters surrounding the main Hawaiian Islands (USA) are exposed to anthropogenic disturbances including fisheries interactions, tourism, naval activities, ocean noise, and contaminants. Although these stocks are managed under the Marine Mammal Protection Act, a demographic assessment has not been undertaken since 2006, and there is currently no information on population trends. We combined regular survey effort with citizen science contributions to estimate apparent survival and annual abundance within each stock using photographs collected between 2000 and 2018. Over this time period, we collected 2818 high-quality identifications of 765 distinctive individuals across all 4 stocks. Analyses of inter-annual movements indicated that individuals exhibit restricted habitat use within stocks, which contributed to non-random sampling. Annual abundance estimates ranged from the 10s to the low 100s. Apparent survival ranged from 0.84 to 0.9, with lower-than-expected estimates in all stocks. Annual abundance estimates declined in 3 of the 4 stocks; however, this decline was not significant for the Kaua‘i/Ni‘ihau and O‘ahu stocks, and may be an artifact of sampling design in all stocks. Given the small population size for these stocks, it is important to closely monitor trends in abundance as a first step in mitigating negative effects of anthropogenic activities. Future efforts should focus on consistent geographic coverage in all stocks to decrease model uncertainty and improve trend assessment.

Abundance and seasonal distribution of the Southern North Carolina estuarine system stock (USA) of common bottlenose dolphins (Tursiops truncatus)

ABSTRACT
 
 Under the U.S. Marine Mammal Protection Act, bottlenose dolphins [Tursiops truncatus] along the United States Atlantic coast are managed as a series of 17 distinct stocks. To determine the status of each stock, the Potential Biological Removal (PBR) is compared with anthropogenic removals, primarily as a result of fisheries bycatch. Estimates of abundance, with associated measures of variance, are required to generate the PBR for each stock. The objectives of the current study were to estimate abundance for the Southern North Carolina Estuarine System Stock (SNCESS) of bottlenose dolphins and to refine the southern boundary of this stock. To meet these objectives, photo-identification surveys were conducted during the summer and winter of 2014 in estuarine and nearshore coastal waters in southern North Carolina. The surveys extended 25km south of the defined southern stock boundary, along the northern South Carolina coast. One mark and one recapture survey were conducted for each season. Each survey was completed in four or five days and covered over 300km of survey tracklines. Dorsal fin images were processed and managed using FinBase, and only images of suitable quality and distinctiveness were used for estimates of abundance. A three-step decision tree was used to assign each dolphin group to either the SNCESS or an adjacent coastal stock, based on sighting location, ranging patterns derived from matches to photo-identification catalogs, and statistical modeling. Only sightings classified as SNCESS were used to estimate stock abundance. Abundance estimates were calculated using three methods: the Chapman modification to the Lincoln-Petersen method, package Rcapture in Program R, and program MARK 6.2. The most parsimonious estimate was generated using the package Rcapture for program R, with the bias corrected M0 model yielding an estimate of 283 dolphins (CV = 0.33, 95% CI 170-396) in the summer of 2014. The distribution of SNCESS dolphins shifted south in the winter and several individuals were observed up to 70km southwest of the currently recognized southern boundary. The results of this study support the current definition of the SNCESS but suggest revisions to the southern boundary. The SNCESS is the smallest bottlenose dolphin stock off the east coast of the USA and is at risk of population decline as a result of fisheries-related mortality.

Updated abundance estimates for common bottlenose dolphins (Tursiops truncatus) inhabiting West Bay, Texas

Abstract West Bay is located along the south‐west portion of Galveston Bay, Texas, and has been delimited as a bay, sound, and estuary (BSE) common bottlenose dolphin (Tursiops truncatus) stock area under the Marine Mammal Protection Act of 1972. The most recent abundance estimate available for the West Bay BSE stock is from 2001 and is considered outdated for management purposes. This study provides an updated abundance estimate using photo‐identification capture‐mark‐recapture (CMR) data collected from West Bay during the winter of 2014 and summer of 2015. The CMR data were analysed using both Pollock's robust design models and Poisson‐log normal mark‐resight models. Differences between the model types are discussed, and the abundance estimates derived from the mark‐resight models are recommended for management use (50.63 [unconditional SE 2.23] in the winter and 44.36 [unconditional SE 1.18] in the summer). Whereas the updated abundance estimates are higher than the previously reported estimates, which varied between 28 and 38, they still indicate that West Bay supports a small population of bottlenose dolphins. As a result, the West Bay BSE stock should be closely monitored for impacts, particularly those related to human caused stressors.

Common bottlenose dolphin (Tursiops truncatus) abundance and distribution patterns in St Andrew Bay, Florida, USA

Abstract Common bottlenose dolphins (Tursiops truncatus) are found in temperate and tropical waters of the world across a wide range of habitats. Along Florida's north‐western coast, this species resides in the bays, sounds, and estuaries (BSE) and coastal (CST) waters of the northern Gulf of Mexico. The National Marine Fisheries Service has identified one CST (Northern Coastal Stock) and seven adjacent BSE dolphin stocks, including the St Andrew Bay BSE Stock. Baseline data are critical to assess the impacts of ongoing and future anthropogenic stressors on these stocks. Currently, there is no comprehensive abundance estimate for the St Andrew Bay BSE Stock, and there are limited data on distribution patterns and site fidelity for this stock. In addition, little is known about the Northern Coastal Stock hypothesized to range from the Big Bend of Florida to the Mississippi River Delta, inclusive of the CST waters adjacent to St Andrew Bay. The goals of this study were to conduct photographic‐identification surveys during 2015 and 2016 to determine abundance, distribution, and site fidelity of common bottlenose dolphins in the St Andrew Bay BSE Stock over four primary periods (July and October 2015, and April and October 2016). St Andrew Bay BSE dolphin abundance was lowest in April 2016 (199, 95% confidence interval [CI] 173–246), followed by July 2015 (249, 95% CI 199–338), and highest in October 2015 (299, 95% CI 259–361) and October 2016 (315, 95% CI 274–378). Few individuals were sighted in both BSE and CST waters (N = 25/353; 7%), and this fact, taken in tandem with limited connections between the BSE and CST environments, suggests that there may be minimal overlap between the St Andrew Bay BSE and Northern Coastal Stocks.

Ranging patterns, spatial overlap, and association with dolphin morbillivirus exposure in common bottlenose dolphins (Tursiops truncatus) along the Georgia, USA coast.

During 2013–2015, an outbreak of dolphin morbillivirus (DMV) occurred in the western North Atlantic, which resulted in the stranding of over 1,600 common bottlenose dolphins (Tursiops truncatus). There are currently five coastal and 10 bay, sound, and estuary dolphin stocks along the U.S. Atlantic coast, yet there is very limited understanding of which stocks were exposed to DMV during the recent outbreak, or how DMV was transmitted across stocks. In order to address these questions, information is needed on spatial overlap and stock interactions. The goals of this project were to determine ranging patterns, prevalence of DMV, and spatial overlap of the South Carolina‐Georgia (SC‐GA) Coastal Stock, and adjacent Southern Georgia Estuarine System (SGES) Stock. During September 2015, a health assessment and telemetry study was conducted in which 19 dolphins were captured, tested for antibodies to DMV, and satellite tagged. Dolphins were classified into one of three ranging patterns (Coastal, Sound, or Estuary) based upon telemetry data. Coastal dolphins (likely members of the SC‐GA Coastal Stock) had a significantly higher prevalence of positive DMV antibody titers (0.67; N = 2/3), than Sound and Estuary dolphins (likely members of the SGES Stock) (0.13; N = 2/16). These results suggest that the SC‐GA Coastal Stock may have experienced greater exposure to DMV as compared to the SGES Stock. However, due to the small size of the SGES Stock and its exposure to high levels of persistent contaminants, this stock may be particularly vulnerable to DMV infection in the future.

Dolphin Liberation in Korea: Is it Beneficial for Conservation?

ABSTRACTThe release of captive dolphins (Tursiops aduncus) from South Korean marine parks, which occurred in 2013, 2015, and 2017, is described and evaluated for its usefulness for conservation of coastal ecosystems. The campaign to free the dolphins created a flagship species, uniting the efforts of national government agencies, local government, and NGOs. However, the Korean management agencies framed the release as a species-specific conservation intervention, while the NGOs brought forward welfare-based demands for the curtailment of cetacean captivity. We argue that the release of the Korean dolphins changed the policy climate, to facilitate two major policy changes: revision of anti-whaling regulations to prohibit capture of cetaceans for exhibit, and improvement of regulations for rescue, rehabilitation, and release of stranded marine animals. Tradeoffs between welfare and conservation concerns are described in terms of risk for the management agency, welfare for the animals, and feasibility of a continued commitment to maintain the viability of the dolphin stock in the wild. The release of captive animals to their native range relies on a single-species conservation focus which poses little risk for management agencies but does not guarantee their continued commitment.

Development of a bioenergetic model for estimating energy requirements and prey biomass consumption of the bottlenose dolphin Tursiops truncatus

The bottlenose dolphin (Tursiops truncatus) is a common species in coastal temperate waters and the ideal candidate for developing a conceptual bioenergetic model given that sufficient information is available to parameterize key input variables. A bioenergetic model was developed to estimate annual energy requirements (MJ/year) and prey biomass consumption (t/year) based on model variations of Field Metabolic Rates (FMR) that included percent of body mass (FMRBodyMass), Kleiber’s scaling equation (FMRKleiber), and measured FMRs (FMRMeasured). Bioenergetic requirements were generated by incorporating the intrinsic uncertainty of input model variables based on assumed or data-driven assignments of sampling distributions. Gompertz growth functions were used to generate body lengths (cm) as a function of age, which once converted to body mass (kg), were used in all calculations. Annual bioenergetic estimates differed across model variations (FMRMeasured>FMRKleiber>FMRBodyMass) and were on average 22%–34% higher in female calves than in male calves, 3%–7% higher in subadult/immature females than in males, and 12%–18% higher in adult males than in non-lactating adult females. Average estimates were ∼72% and 31%–34% higher in lactating adult females compared to non-lactating adult females and adult males, respectively. Annual bioenergetic requirements for ≥2year old dolphins normalized by body mass were FMRMeasured: 205±29MJ/kg/year and 34±5kg/kg/year, FMRKleiber: 151±29MJ/kg/year and 22±5kg/kg/year, and FMRBodyMass: 138±38MJ/kg/year and 20±5kg/kg/year. When applying the bioenergetic model to the US bottlenose dolphin stock with the largest dolphin abundance (n=950), estimates of annual bioenergetic requirements were 2040–3050 MJ*104/year and 2900–5070t/year. While the existing information provides the foundation to develop a bioenergetic model specific for bottlenose dolphins, improvements of this and related models require additional data on field measurements of metabolic rates, cost of lactation, caloric intake and metabolization efficiency. This bioenergetic model could be used to better understand the complex ecological and trophic interactions of bottlenose dolphins with their prey populations, to evaluate the role of disturbance on bioenergetic requirements, and to inform management and conservation efforts.

Using Genome-Wide SNPs to Detect Structure in High-Diversity and Low-Divergence Populations of Severely Impacted Eastern Tropical Pacific Spinner (Stenella longirostris) and Pantropical Spotted Dolphins (S. attenuata)

Millions of spinner (Stenella longirostris) and pantropical spotted dolphins (Stenella attenuata) died since the 1960’s as bycatch in tuna nets in the eastern tropical Pacific Ocean. Despite three decades of protection, they show little-to-no sign of recovery (although recent fisheries-independent abundance estimates are not available). In efforts to establish biologically meaningful management boundaries for recovery, endemic subspecies and multiple stocks have been proposed. However, genetic differentiation among most of these units has been difficult to identify, possibly due to low statistical power stemming from large historical abundances, ongoing gene flow, and recent divergence. We tested for genetic structure at multiple hierarchical levels by analyzing the largest dataset to date brought to bear on these questions. Single nucleotide polymorphisms (SNPs) were collected from nuclear DNA regions associated with the restriction enzyme site PstI from 72 spinner dolphins and 58 pantropical spotted dolphins using genotype-by-sequencing (GBS). Our results support the current subspecies for both species and indicate stock-level separation for Tres Marias spinner dolphins and the two offshore pantropical spotted dolphin stocks in this area. Although bycatch has been reduced a small fraction of pre-protection levels, incidental mortality continues to impact these populations. Our results are important for the ongoing management and recovery of these highly-impacted pelagic dolphins in the eastern tropical Pacific Ocean.

Evidence for Distinct Coastal and Offshore Communities of Bottlenose Dolphins in the North East Atlantic

Bottlenose dolphin stock structure in the northeast Atlantic remains poorly understood. However, fine scale photo-id data have shown that populations can comprise multiple overlapping social communities. These social communities form structural elements of bottlenose dolphin (Tursiops truncatus) populations, reflecting specific ecological and behavioural adaptations to local habitats. We investigated the social structure of bottlenose dolphins in the waters of northwest Ireland and present evidence for distinct inshore and offshore social communities. Individuals of the inshore community had a coastal distribution restricted to waters within 3 km from shore. These animals exhibited a cohesive, fission-fusion social organisation, with repeated resightings within the research area, within a larger coastal home range. The offshore community comprised one or more distinct groups, found significantly further offshore (>4 km) than the inshore animals. In addition, dorsal fin scarring patterns differed significantly between inshore and offshore communities with individuals of the offshore community having more distinctly marked dorsal fins. Specifically, almost half of the individuals in the offshore community (48%) had characteristic stereotyped damage to the tip of the dorsal fin, rarely recorded in the inshore community (7%). We propose that this characteristic is likely due to interactions with pelagic fisheries. Social segregation and scarring differences found here indicate that the distinct communities are likely to be spatially and behaviourally segregated. Together with recent genetic evidence of distinct offshore and coastal population structures, this provides evidence for bottlenose dolphin inshore/offshore community differentiation in the northeast Atlantic. We recommend that social communities should be considered as fundamental units for the management and conservation of bottlenose dolphins and their habitat specialisations.

Abundance and Survival Rates of the Hawai’i Island Associated Spinner Dolphin (Stenella longirostris) Stock

Reliable population estimates are critical to implement effective management strategies. The Hawai’i Island spinner dolphin (Stenella longirostris) is a genetically distinct stock that displays a rigid daily behavioural pattern, foraging offshore at night and resting in sheltered bays during the day. Consequently, they are exposed to frequent human interactions and disturbance. We estimated population parameters of this spinner dolphin stock using a systematic sampling design and capture–recapture models. From September 2010 to August 2011, boat-based photo-identification surveys were undertaken monthly over 132 days (>1,150 hours of effort; >100,000 dorsal fin images) in the four main resting bays along the Kona Coast, Hawai’i Island. All images were graded according to photographic quality and distinctiveness. Over 32,000 images were included in the analyses, from which 607 distinctive individuals were catalogued and 214 were highly distinctive. Two independent estimates of the proportion of highly distinctive individuals in the population were not significantly different (p = 0.68). Individual heterogeneity and time variation in capture probabilities were strongly indicated for these data; therefore capture–recapture models allowing for these variations were used. The estimated annual apparent survival rate (product of true survival and permanent emigration) was 0.97 SE±0.05. Open and closed capture–recapture models for the highly distinctive individuals photographed at least once each month produced similar abundance estimates. An estimate of 221±4.3 SE highly distinctive spinner dolphins, resulted in a total abundance of 631±60.1 SE, (95% CI 524–761) spinner dolphins in the Hawai’i Island stock, which is lower than previous estimates. When this abundance estimate is considered alongside the rigid daily behavioural pattern, genetic distinctiveness, and the ease of human access to spinner dolphins in their preferred resting habitats, this Hawai’i Island stock is likely more vulnerable to negative impacts from human disturbance than previously believed.

Reproductive seasonality of a recently designated bottlenose dolphin stock near Charleston, South Carolina, U.S.A

AbstractReproductive seasonality of common bottlenose dolphins (Tursiops truncatus) can be affected by numerous ecological and behavioral factors. In 2009, stock boundaries were revised to recognize a Charleston Estuarine System Stock (CESS) of bottlenose dolphins in Charleston, South Carolina. The CESS is a well‐studied population with long‐term data collected from photo‐identification and stranding studies. From 2004 to 2008, a systematic mark‐recapture photo‐identification study was conducted in the Charleston Estuary to estimate population size of the CESS. Sightings data from this photo‐identification study coupled with strandings data (1993–2008) were analyzed to determine the reproductive seasonality of this local population. Both neonate sightings and strandings depicted a primary season of reproduction in the spring into early summer with a small peak in neonate sightings in early autumn, and were significantly different from circular uniform and Von Mises distributions (strandings: P < 0.01, V = 2.8644; sightings: P < 0.01, V = 3.2302). This study increases the knowledge of seasonal reproductive patterns of estuarine stocks of bottlenose dolphin stocks in the southeastern United States. The results will also help wildlife managers detect unusual neonate mortality events, and provide information about critical habitat relevant for evaluating and mitigating coastal development projects.

Integrating multiple techniques to identify stock boundaries of common bottlenose dolphins (Tursiops truncatus)

ABSTRACT Under US policy, a stock is a group of animals shown to be demographically independent from other such groups. Evidence of demographic delineations for marine mammal stocks is gained primarily through genetic analysis. However, additional techniques play an important role in determining fine‐scale ranging patterns that can be used to define a stock's geographic boundaries. In cases where genetic studies have not yet been performed, movement and ranging pattern data are essential in identifying a targeted geographic region for tissue sampling and genetic studies. Photo‐identification surveys, vessel‐based radio telemetry, automated radio telemetry systems (ARTS), and satellite‐linked telemetry are sampling techniques that have been used to determine common bottlenose dolphin ranging patterns and provide detailed insight into stock boundaries. The results of near simultaneous use of these sampling techniques are compared using data from a case study of bottlenose dolphins within the estuaries of southern Georgia. Satellite‐linked and radio telemetry were determined to be useful sampling techniques for identification of short‐term ranging patterns. Satellite‐linked telemetry had the second lowest cost per location ($122) and identified dolphin ranging patterns within and outside of the study area boundaries. Vessel‐based radio telemetry was more costly ($195 per location) and had relatively limited tracking coverage. However, this sampling technique permitted visual observations of animal and tag condition. The combination of vessel‐based radio telemetry and ARTS, which had the lowest cost per location ($34), was an effective method for determining ranging patterns of tagged individuals within and outside of the study area. Photo‐identification surveys, relative to satellite‐linked and radio telemetry, were not as efficient for determination of targeted individuals’ short‐term ranging patterns and had the highest cost per location of the four sampling techniques ($292). However, photo‐identification is more effective than any other technique for compiling data on large numbers of individuals within a designated study area. Photo‐identification surveys are essential for long‐term monitoring and provide additional insight into dolphin stock structure that cannot be determined through telemetry alone. Copyright © 2013 John Wiley & Sons, Ltd.

Passive acoustic recording to build acoustic catalogs to remotely monitor resident individuals and the health of the dolphin population in the Indian River Lagoon system in Florida

Harbor Branch Oceanographic Institute at Florida Atlantic University together with, NOAA's National Ocean Service/Center for Coastal Environmental Health & Biomolecular Research initiated a research program designed to assess environmental and anthropogenic stressors that may affect the health and long-term viability of bottlenose dolphin populations inhabiting coastal regions of Florida and South Carolina This collaborative program is known as the Health and Environmental Risk Assessment Project. The project involves the capture, sampling, and release of selected wild dolphin stocks to allow comprehensive health screenings by collecting and analyzing a variety of biomedical samples and associated data. During Indian River Lagoon (IRL) dolphin population assessment, the acoustic behavior of 33 individual dolphins was recorded using synchronized DARP buoys configured with directional and omnidirectional hydrophones. Vocalizations were recorded during the capture, holding and health sampling phases. Acoustic levels and behavior during ABR hearing measurements were also monitored. The whistle contours of 23 individuals have been identified and will be added to the photo-identification and genetic catalog maintained for the IRL population. These signature whistles will be used to train a network of remote acoustic sensors to monitor the distribution, social interactions, and habitat utilization of cataloged dolphins in the IRL. Authorized NMFS permit 14352-01.

Defining bottlenose dolphin (Tursiops truncatus) stocks based on environmental, physical, and behavioral characteristics

AbstractThe population structure of bottlenose dolphins, Tursiops truncatus, along the U.S. Atlantic coast has recently been redefined from one homogenous population into five coastal stocks. Local studies indicate even finer structure, primarily based on isolation of dolphins inhabiting estuaries. We identified population structuring of non‐estuarine coastal bottlenose dolphins during a study in New Jersey, the northern range along the Atlantic Coast. Using photo‐identification and distribution survey results, an analysis identified two major clusters of individuals significantly separated by five variables (distance from shoreline, group size, occurrence of the barnacle Xenobalanus globicipitis, avoidance behavior, and individual coloration). Sightings assigned to cluster 1 occurred in nearshore shallow waters (0–1.9 km, x̄= 3.5 m), and those assigned to cluster 2 occurred further offshore in deeper waters (1.9–6 km, x̄= 9.5 m). Only eight of 194 individuals (4%) were identified in both regions. Collectively, this suggests an occurrence of two stocks that are spatially, physically, and behaviorally distinguishable over a small distance. These results indicate that complexity in Tursiops population structure is not limited to latitudinal gradients or barriers created by estuarine habitats, but also by partitioning of habitat as a function of distance from shore and depth over small distances.

Plastic ingestion in Franciscana dolphins, Pontoporia blainvillei (Gervais and d’Orbigny, 1844), from Argentina

Plastic debris (PD) ingestion was examined in 106 Franciscana dolphins (Pontoporia blainvillei) incidentally captured in artisanal fisheries of the northern coast of Argentina. Twenty-eight percent of the dolphins presented PD in their stomach, but no ulcerations or obstructions were recorded in the digestive tracts. PD ingestion was more frequent in estuarine (34.6%) than in marine (19.2%) environments, but the type of debris was similar. Packaging debris (cellophane, bags, and bands) was found in 64.3% of the dolphins, with a lesser proportion (35.7%) ingesting fishery gear fragments (monofilament lines, ropes, and nets) or of unknown sources (25.0%). PD ingestion correlated with ontogenetic changes in feeding regimes, reaching maximum values in recently weaned dolphins. Because a simultaneous increase in gillnet entanglement and the bioaccumulation of heavy metals take place at this stage, the first months after trophic independence should be considered as a key phase for the conservation of Franciscana dolphin stocks in northern Argentina.

Life or death in the fast lane: dolphin mother‐calf separation during tuna purse‐seine sets

Dolphin stocks in the eastern tropical Pacific Ocean, reduced 70–80% due to fishery‐related mortality 1960–1990, are not recovering despite near‐zero fishery mortality since 1991. Because mortality records show that 75–95% of killed lactating females are not killed with a nursing calf, the fishing process appears to separate mothers and calves, so that unobserved mortality of calves may be important to the lack of stock recovery. A suite of studies investigated this question from a number of perspectives, including review of physiological and behavioral development in dolphin calves, mathematical analysis of dolphin drafting hydrodynamics, and empirical studies of swim development in dolphin calves from birth through two years. In toto, these studies show that the sustained high‐speed swimming and accompanying aerial respirations characteristic of dolphins evading purse‐seine sets creates conditions under which dolphin calves will have great difficulty sustaining a drafting relationship with the mother, and that developmental limits are likely to prevent calves from maintaining an association with the mother while swimming independently. Effects are strongly age‐related, with youngest calves at greatest risk for separation.

Fishery Effects on Dolphins Targeted by Tuna Purse-seiners in the Eastern Tropical Pacific Ocean

Dolphins in the eastern tropical Pacific Ocean (ETP), particularly spotted (Stenella attenuate) and spinner (Stenella longirostris) dolphins, are subject to fishery-induced stress due to chase and encirclement by tuna purse-seiners intent on capturing the large yellowfin tuna that are frequently found associated with dolphin schools in this area of the Pacific Ocean. The direct, observed mortality of dolphins in the fishing nets has decreased over the years from several hundred thousand annually during the early 1960’s when the fishing practice originated, to less than 5000 dolphins annually (thought to be a biologically insignificant level) since the early 1990s. Despite the decrease in observed mortality, the dolphin populations have not been recovering as expected. In an effort to determine whether fishery-related stress may be contributing to this lack of recovery, through unobserved effects on survival or reproduction, a variety of studies have been and continue to be conducted examining various aspects of interactions between ETP dolphins and the tuna purse-seine fishery. These studies include a review of current knowledge of stress physiology in mammals, a necropsy program to examine dolphins killed during purse-seining operations, a chase-recapture experiment, and various analyses of existing (historical) data which have led to ongoing studies of fishery effects on mother-calf pairs, ETP dolphin reproductive biology, and analyses of dolphin school composition. The effect of noise has not been addressed directly in these studies, but potentially contributes to fishery-related stress in terms of initiating the significant and prolonged evasion response typical of dolphin schools reacting to tuna purse-seiners in the ETP. Although studies completed to date have not provided a definitive answer to whether fishery-induced stress is a significant factor in the lack of dolphin stock recovery in the ETP, it is possible that at least some adults, and probably many young dolphins, are negatively affected by interactions with tuna purseseine fishing operations.