Common Bottlenose Dolphins At Risk Of Bycatch

Introduction

Common bottlenose dolphins (Tursiops truncatus) inhabiting the coastal and estuarine waters of North Carolina face several human-related threats, including pollution, habitat alteration, vessel strikes, illegal feeding, recreational fishing gear, and accidental capture in commercial fisheries. One of the most serious of these threats is fisheries bycatch, which occurs when an animal that fishers do not intend to catch becomes entangled, hooked, trapped, or otherwise captured by fishing equipment.

Gillnets present a particular risk because they consist of nearly invisible mesh panels designed to capture fish by their gills. Dolphins may accidentally swim into these nets while travelling or feeding. They may also approach fishing gear to capture concentrated fish, remove prey already caught in the net, or feed on discarded catch. These interactions can lead to entanglement, drowning, serious wounds, infection, reduced swimming ability, or death. NOAA Fisheries identifies commercial and recreational fishing gear as one of the principal human-caused threats to bottlenose dolphins.

Directly observing a dolphin becoming entangled is uncommon because fisheries operate across large areas and only a relatively small proportion of fishing activity is monitored by trained observers. Consequently, stranded dolphins provide an additional source of evidence. A carcass recovered on a beach may display attached fishing gear, linear wounds, impressions around the fins, cuts, or other injuries consistent with entanglement.

A major study by Barbie L. Byrd and Aleta A. Hohn examined bottlenose dolphin strandings recorded in North Carolina from 1997 through 2012. The researchers investigated whether age, sex, season, geographic area, habitat, and changes in fishery regulation influenced the likelihood that a stranded dolphin showed evidence of a fisheries interaction. Their findings demonstrated that older calves and subadult dolphins experienced a substantially greater relative risk than adults and young-of-year animals. The study provides important evidence that bycatch does not affect every age group equally and that young dolphins surviving beyond their first year may be particularly vulnerable (Byrd & Hohn, 2017).

What Is Fisheries Bycatch?

Bycatch refers to the unintended capture, injury, or death of non-target animals during fishing operations. A fishing vessel may be targeting species such as spot, Spanish mackerel, smooth dogfish, or spiny dogfish while unintentionally capturing a dolphin, sea turtle, seabird, shark, or other marine animal.

Bycatch does not necessarily result from deliberate wrongdoing by fishers. Commercial fisheries provide food, income, and employment, while most fishers do not intend to harm marine mammals. The problem develops when fishing effort, gear characteristics, dolphin distribution, and animal behavior overlap in ways that create an entanglement risk.

Bottlenose dolphins may interact with several forms of fishing gear, including:

• Gillnets
• Trawls
• Pound nets
• Seines
• Crab-pot and trap lines
• Longlines
• Recreational hook-and-line equipment

Gillnets are especially relevant in North Carolina because they are widely used in estuarine and coastal fisheries. A gillnet may be anchored in one place, allowed to drift, or set vertically through the water. Because the mesh can be difficult to detect, a dolphin may encounter it before having sufficient time or space to avoid it.

A dolphin entangled underwater may be unable to reach the surface to breathe. Even when it escapes, it may retain fishing line or netting around its fins, mouth, tail stock, or body. These materials can gradually cut into the skin, interfere with movement, impair feeding, and create life-threatening infections.

Fisheries Interaction and Depredation

Not every dolphin-fishery interaction is accidental contact during normal travel. Bottlenose dolphins are intelligent and adaptable predators capable of learning that fishing vessels and nets provide access to concentrated prey.

When dolphins remove fish or bait from fishing equipment, the behavior is known as depredation. A dolphin may take fish directly from a net, hook, or line or may follow a trawler to consume discarded or disturbed prey. Depredation can provide an immediate feeding opportunity, but it also brings dolphins dangerously close to equipment capable of injuring or killing them.

The behavior may spread through social learning. Young dolphins can observe their mothers or other group members approaching boats and fishing gear. NOAA warns that feeding wild dolphins, whether intentional or indirect, can teach them to associate people and vessels with food. This makes the animals more likely to approach fishing activity and therefore more vulnerable to entanglement, hook ingestion, vessel strikes, and retaliation from frustrated fishers.

However, depredation should not be assumed in every bycatch event. A dolphin may fail to detect a net, misjudge its position, become trapped while chasing fish, or encounter gear during ordinary movement. Since most entanglements are never directly witnessed, the precise behavior leading to an individual death often remains unknown.

Why Stranding Records Are Important

A stranding occurs when a live or dead marine mammal is found on a beach, in shallow water, floating near shore, or in another location where it cannot return to its normal habitat without assistance. Stranding-network personnel examine the animal, record measurements, document injuries, collect samples, and, when possible, determine whether human activity contributed to the event.

In the North Carolina study, researchers used evidence from stranded animals because fishery-observer programs documented relatively few bottlenose dolphin captures. Low numbers of observed events do not necessarily mean that bycatch is rare throughout the entire fishery. Observer programs monitor only a fraction of fishing trips, and an entangled dolphin may escape from a net, sink after death, decompose offshore, or strand in a place where it is never recovered.

Stranding records can therefore reveal patterns that vessel-based observations alone may miss. They may show whether particular ages, seasons, or areas appear more frequently among dolphins with entanglement injuries.

Nevertheless, stranding data cannot provide a complete count of all dolphins killed in fishing gear. Researchers do not know what proportion of dead dolphins eventually reaches the shore. Ocean currents, weather, decomposition, scavenging, distance from land, and carcass buoyancy all influence recovery. Stranding records are therefore best used as indicators of relative patterns rather than exact measures of total mortality.

The Purpose of the North Carolina Study

Byrd and Hohn sought to determine whether the relative risk of a stranded dolphin showing evidence of fisheries interaction differed according to sex or age class. They also considered several variables already known to influence stranding and fishing patterns:

• Season
• Geographic area
• Coastal or estuarine habitat
• Sex
• Age class
• Time period

The researchers were especially interested in changes associated with the spiny dogfish fishery. Gillnet effort directed at spiny dogfish declined sharply following the implementation of state and federal fishery-management measures in November 2000. The Bottlenose Dolphin Take Reduction Plan was subsequently implemented in 2006 to reduce serious injury and mortality in several commercial fisheries.

The study therefore divided the data into three periods:

These divisions allowed the researchers to examine whether the proportion of fisheries-interaction strandings changed alongside important regulatory developments.

How the Strandings Were Classified

Researchers first examined each stranding for evidence of human interaction. The animals were assigned to one of three main categories:

Fisheries Interaction

A dolphin was categorized as positive for fisheries interaction when investigators identified attached gear, characteristic entanglement wounds, or other reliable physical evidence connecting the animal with fishing equipment.

No Human Interaction

This category was used when the carcass was sufficiently complete and well preserved for investigators to conclude that there was no visible evidence of human interaction.

Could Not Be Determined

The designation “could not be determined,” abbreviated as CBD, was used when decomposition, scavenger damage, missing body parts, or other limitations prevented investigators from determining whether human activity had affected the animal.

A fourth group included evidence of human interaction unrelated to fisheries, such as propeller injuries or mutilation without entanglement wounds.

The CBD category is particularly important because many stranded carcasses are too decomposed for a reliable conclusion. A dolphin without recorded evidence of entanglement cannot automatically be assumed to have died naturally if its body was not in a condition that allowed a complete examination.

The Number of Strandings Recorded

Between 1997 and 2012, the North Carolina stranding network recovered 1,368 bottlenose dolphin strandings. This represented an annual average of 85.5 animals.

The initial classifications were as follows:

Among the 547 animals for which investigators could reach a human-interaction determination, 229, or 42 percent, showed evidence of fisheries interaction. Another 274 showed no evidence of human interaction, while 44 displayed evidence of another form of human involvement.

The study did not include all 503 dolphins originally classified as either fisheries-interaction or no-human-interaction cases in its final statistical analysis. Animals were excluded when sex could not be determined, body length was incomplete or estimated, only a partial carcass was recovered, or the dolphin was an extremely small neonate.

Dolphins measuring less than 125 centimetres were excluded because very young calves experience naturally high mortality, particularly during spring. Including this seasonal pulse of neonatal deaths might have distorted comparisons between natural and fisheries-related mortality.

After the exclusions, the final analytical sample consisted of 361 dolphins:

• 191 with evidence of fisheries interaction
• 170 with no evidence of human interaction

The difference between the 229 original fisheries-interaction cases and the 191 included cases is therefore not an inconsistency. It resulted from the study’s inclusion criteria.

How Age Classes Were Defined

The researchers used body length to assign dolphins to four age-related groups. These categories reflected expected developmental and behavioral differences.

Very small neonates measuring less than 125 centimetres were removed from the statistical analysis.

The use of length as an indicator of age is not exact because growth differs among individuals and between males and females. Nevertheless, it provided a practical method for evaluating broad developmental groups when exact ages were unavailable.

The Statistical Method

The researchers used a generalized linear model with a logistic link to estimate the probability that a stranding would be categorized as a fisheries interaction rather than as having no evidence of human involvement.

A logistic model is suitable when the outcome has two possibilities. In this study, the outcome was either:

1. Positive for fisheries interaction, or
2. No evidence of human interaction.

Because the final sample was not large enough to include every predictor in one complex model, the researchers first tested variables separately. Factors that showed statistically meaningful relationships were then considered in the generalized linear model.

This approach allowed the researchers to evaluate whether the observed pattern was associated with age, sex, season, location, habitat, or management period rather than relying only on raw counts.

Age Was the Strongest Predictor

Age class was the clearest predictor of fisheries-interaction risk. Older calves and subadults were significantly more likely to strand with evidence of fisheries interaction than adults or young-of-year dolphins.

Depending on the season, a stranded older calf or subadult was approximately 1.5 to 3.5 times as likely to show evidence of fisheries interaction as a stranded adult or young-of-year animal. The difference remained significant even after sex was considered in the statistical model.

This finding is particularly important because older calves and subadults have already survived the earliest stage of life, when natural mortality tends to be highest. Under normal conditions, dolphins that survive the first years of life should enter a period of comparatively lower natural mortality. The authors concluded that fishery-related mortality among these age groups “appears to exceed natural mortality,” at least according to the stranding evidence available (Byrd & Hohn, 2017, p. 564).

Several explanations may account for their increased vulnerability. Young dolphins may:

• Have less experience identifying or avoiding nets
• Explore unfamiliar objects or fishing operations
• Be developing independent feeding strategies
• Remain near their mothers while also moving more independently
• Attempt to capture fish concentrated around nets
• Learn depredation behavior from other dolphins
• Lack the body strength or experience required to escape entanglement

These explanations remain hypotheses. The study did not follow identified free-swimming dolphins to determine exactly how individual behavior changed with age. It established a pattern in recovered strandings rather than directly observing the events that caused each entanglement.

Season Also Influenced Risk

Season was another significant predictor. The statistical analysis found that bycatch risk was higher in spring than in summer. Differences between other seasonal comparisons were not statistically significant.

Seasonal patterns likely reflect changes in both dolphin distribution and fishing activity. Different fisheries operate at different times of the year, and target species move according to temperature, reproduction, and prey availability. Dolphins also change their movements and feeding behavior seasonally.

Spring risk may therefore rise when particular dolphin stocks overlap with gillnet fisheries targeting species such as spiny dogfish or other coastal fish. The study’s finding does not mean that bycatch occurs only in spring. Fisheries-interaction strandings were recorded throughout the year, but the relative probability differed among seasons.

Sex Did Not Determine Bycatch Risk

More male than female dolphins were recovered during the combined second and third study periods. However, sex did not significantly predict whether a stranding would show evidence of fisheries interaction.

This distinction is important. A larger number of male strandings does not necessarily mean that male dolphins are more likely to become entangled. The higher number may reflect sex differences in the population, behavior, distribution, natural mortality, or the likelihood of a carcass reaching shore.

When age and sex were considered together, age remained statistically significant while sex did not. Both male and female older calves and subadults showed the elevated fisheries-interaction pattern.

Geographic Area Was Not a Reliable Predictor

Initial comparisons suggested differences among geographic areas. However, one area contained only a small number of strandings. When that area was removed to test whether the result was caused by its limited sample size, the geographic effect was no longer statistically significant.

The researchers therefore concluded that area was not a reliable predictor of bycatch risk in the final analysis. This was somewhat unexpected because dolphin abundance and gillnet effort differ along the North Carolina coast.

One possible explanation is that several dolphin stocks and fisheries overlap across multiple regions. Carcasses can also move with tides and currents before being recovered, meaning that the stranding location may not be the location where the animal became entangled.

Changes Across the Three Time Periods

Bycatch risk differed significantly among the three management periods. It was higher during TP1 than TP2. This decrease coincided with regulations that sharply reduced the North Carolina spiny dogfish gillnet fishery beginning in November 2000.

The result is important because it suggests that reducing fishing effort in a high-risk fishery can reduce dolphin mortality. Previous studies had similarly found that declining spiny dogfish fishing activity was accompanied by reductions in observer-documented bycatch and strandings with entanglement evidence.

Risk increased slightly during TP3, although it was not statistically different from both earlier periods. The study proposed that renewed spiny dogfish activity may have contributed to this increase even after the Bottlenose Dolphin Take Reduction Plan was introduced. Two of four observer-documented North Carolina gillnet entanglements after 2009 involved gear targeting spiny dogfish. However, the study could not establish that increased dogfish effort directly caused the TP3 pattern.

The Bottlenose Dolphin Take Reduction Plan

NOAA Fisheries implemented the Bottlenose Dolphin Take Reduction Plan to reduce incidental mortality and serious injury in commercial fisheries along the western North Atlantic coast.

The plan includes measures such as:

• Seasonal gillnet restrictions
• Limits on the length of fishing gear
• Requirements concerning how close fishers must remain to their gear
• Restrictions on overnight gillnet sets in designated areas
• Gear modifications for particular pound-net fisheries
• Outreach and cooperation with the fishing industry

The plan was developed through collaboration among NOAA, commercial fishers, scientists, conservation organizations, fishery-management bodies, and state and federal agencies.

The North Carolina study does not prove that the plan failed. Fishing effort, target species, net size, soak duration, dolphin distribution, and observer coverage all changed during the study period. The authors concluded only that a slight increase in relative bycatch risk occurred during TP3 and that additional investigation was required.

Limitations of the Study

The findings should be interpreted as evidence of relative risk among recovered strandings, not as a precise estimate of the probability that any living dolphin will become entangled.

Several limitations are important.

First, only a proportion of dolphins that die at sea eventually strand and are recovered. Second, carcass condition affects whether injuries can be identified. Sixty percent of the original 1,368 strandings were classified as could not be determined.

Third, identifying a fisheries interaction may require only one recognizable lesion, while determining that no human interaction occurred requires a relatively complete and well-preserved body. This could influence comparisons between the two categories.

Fourth, the precise fishing gear responsible usually could not be identified. Dolphins in North Carolina interact with several fisheries, so a stranding with an entanglement wound cannot always be attributed to a particular target species or gear configuration.

Fifth, the analysis assumed that dolphins killed in fishing gear and dolphins dying from non-human causes were equally likely to strand. That assumption cannot be tested easily because currents, depth, weather, and cause of death may affect carcass recovery differently. Byrd and Hohn explicitly acknowledged these potential biases.

Conservation Significance

The loss of older calves and subadults may have long-term population consequences. Bottlenose dolphins reproduce slowly. Females carry a calf for approximately 12 months, nurse it for an extended period, and generally give birth only once every three to six years. Removing young dolphins before they reach reproductive maturity reduces the number that can later contribute to population growth.

This issue is especially important in North Carolina because several estuarine and coastal stocks use overlapping waters. Some local stocks contain relatively small numbers of dolphins, and even a limited amount of preventable mortality may be biologically significant.

NOAA’s later Mid-Atlantic analysis demonstrated the difficulty of estimating total bycatch when observer coverage is low. From 2007 to 2015, observers documented only five bottlenose dolphin takes in commercial gillnet fisheries, yet model-based estimates suggested that actual mortality could be considerably higher. Observer coverage in some internal and state waters was extremely limited, reinforcing the value of stranding records as a complementary monitoring tool.

Ways to Reduce Dolphin Bycatch

Effective conservation should protect dolphin populations while recognizing the economic importance of coastal fishing communities. Potential strategies include:

Improving Observer Coverage

Greater coverage would provide more accurate information about when, where, and how entanglements occur. Electronic cameras and other monitoring systems may supplement human observers where appropriate.

Adjusting Fishing Seasons and Areas

Temporary restrictions can reduce overlap when dolphins and high-risk fisheries occur in the same location and season. The decline following reduced spiny dogfish effort suggests that such measures can be effective.

Limiting Net Length and Soak Time

Shorter nets and reduced soak times may allow fishers to detect and respond to entanglements more quickly. Gear left unattended overnight may pose a greater risk because trapped animals cannot be released promptly.

Developing Safer Gear

Researchers and fishers can test net modifications, acoustic warning devices, weak links, reflective materials, and alternative fishing methods. A successful modification must reduce dolphin injury without creating unacceptable economic losses or additional risks to other wildlife.

Preventing Dolphin Feeding

Members of the public should never feed wild dolphins or discard bait in ways that encourage animals to approach fishing vessels. Feeding changes natural behavior and may increase the likelihood that dolphins will steal bait or catch from fishing gear.

Supporting Stranding Networks

Rapid examination of stranded dolphins improves the quality of data. Networks require trained personnel, veterinary expertise, transportation, laboratory capacity, and long-term funding.

Working With Fishers

Fishers possess detailed knowledge of gear, currents, target species, and animal behavior. Conservation measures are more likely to be practical and effective when the fishing community participates in their development, testing, and evaluation.

Conclusion

Common bottlenose dolphins in North Carolina face a measurable risk of injury and death through fisheries bycatch, particularly in gillnets. Although direct observations of entanglement are uncommon, stranding records provide valuable evidence about the animals most affected and the conditions under which risk changes.

Between 1997 and 2012, 1,368 bottlenose dolphin strandings were recovered in North Carolina. Sixty percent could not be evaluated conclusively for human interaction. Among the remaining animals, 229 displayed evidence of fisheries interaction. After applying strict inclusion criteria, researchers analyzed 191 fisheries-interaction strandings and 170 strandings with no evidence of human involvement.

Age class was the strongest predictor. Older calves and subadults were approximately 1.5 to 3.5 times more likely than adults or young-of-year dolphins to strand with evidence of fisheries interaction. Spring presented a higher relative risk than summer, while sex and geographic area did not reliably predict bycatch after other factors and sample-size limitations were considered.

The decline in bycatch risk following the sharp reduction of the spiny dogfish gillnet fishery demonstrates that fishery-management measures can contribute to marine-mammal protection. At the same time, the slight increase observed after 2006 shows that conservation must be continuously evaluated as fishing practices, regulations, and dolphin distributions change.

Stranding data cannot reveal every entanglement or calculate total mortality precisely. However, they clearly indicate that fisheries interactions are an important source of human-caused mortality. Protecting bottlenose dolphins will require improved monitoring, carefully designed fishing regulations, safer equipment, strong stranding-response networks, public education, and sustained cooperation with commercial and recreational fishers.

References

Byrd, B. L., and Hohn, A. A. (2017). Differential risk of bottlenose dolphin (Tursiops truncatus) bycatch in North Carolina, USA. Aquatic Mammals, 43(5), 558–569. doi:10.1578/AM.43.5.2017.558.

Byrd, B. L., Hohn, A. A., Lovewell, G. N., Altman, K. M., Barco, S. G., Friedlaender, A., Harms, C. A., McLellan, W. A., Moore, K. T., Rosel, P. E., and Thayer, V. G. (2014). Strandings as indicators of marine mammal biodiversity and human interactions off the coast of North Carolina. Fishery Bulletin, 112(1), 1–23.

Friedlaender, A. S., McLellan, W. A., and Pabst, D. A. (2001). Characterising an interaction between coastal bottlenose dolphins (Tursiops truncatus) and the spot gillnet fishery in southeastern North Carolina, USA. Journal of Cetacean Research and Management, 3(3), 293–303.

Hohn, A. A., Gorgone, A. M., Byrd, B. L., and Eguchi, T. (2022). Patterns of association and distribution of estuarine-resident common bottlenose dolphins (Tursiops truncatus) in North Carolina, USA. PLOS ONE, 17(8), e0270057. doi:10.1371/journal.pone.0270057.

Lyssikatos, M. C., and Garrison, L. P. (2018). Common bottlenose dolphin Tursiops truncatus gillnet bycatch estimates along the US Mid-Atlantic coast 2007–2015. NOAA Northeast Fisheries Science Center Reference Document 18-07.

National Marine Fisheries Service. (2006). Environmental assessment for the Bottlenose Dolphin Take Reduction Plan. National Oceanic and Atmospheric Administration.

NOAA Fisheries. (2025). Common bottlenose dolphin. National Oceanic and Atmospheric Administration.

Palka, D. L., and Rossman, M. C. (2001). Bycatch estimates of coastal bottlenose dolphin Tursiops truncatus in US Mid-Atlantic gillnet fisheries for 1996 to 2000. NOAA Northeast Fisheries Science Center Reference Document 01-15.

Read, A. J., Waples, D. M., Urian, K. W., and Swanner, D. (2003). Fine-scale behaviour of bottlenose dolphins around gillnets. Proceedings of the Royal Society B, 270, S90–S92.

Read, A. J., Wells, R. S., Hohn, A. A., and Scott, M. D. (1993). Patterns of growth in wild bottlenose dolphins, Tursiops truncatus. Journal of Zoology, 231(1), 107–123. doi:10.1111/j.1469-7998.1993.tb05356.x.

Editorial Staff

Academic Master Education Team is a group of academic editors and subject specialists responsible for producing structured, research-backed essays across multiple disciplines. Each article is developed following Academic Master’s Editorial Policy and supported by credible academic references. The team ensures clarity, citation accuracy, and adherence to ethical academic writing standards

Content reviewed under Academic Master Editorial Policy.

This author does not have any more posts.