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Northeast Fisheries Science Center Reference Document 08-04

Mortality and Serious Injury Determinations for Baleen Whale Stocks along the United States Eastern Seaboard and Adjacent Canadian Maritimes, 2002-2006

Allison H. Glass, Timothy V.N. Cole, Mendy Garron, Richard L. Merrick, Richard M. Pace III
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543

Web version posted February 14, 2008

Citation: Glass AH, Cole TVN, Garron M, Merrick RL, Pace RM III. 2008. Mortality and Serious Injury Determinations for Baleen Whale Stocks Along the United States Eastern Seaboard and Adjacent Canadian Maritimes, 2002-2006. US Dep Commer, Northeast Fish Sci Cent Doc. 08-04; 18 p.

Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.

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The Northeast Fisheries Science Center (NEFSC) has developed criteria to evaluate reports of human-caused injury and mortality to large whales. Serious injury is defined as an injury that is likely to lead to death. The criteria minimize the identification of false positive human-caused mortalities and serious injuries, and therefore provide a minimum value of human impact to whale stocks. This report describes determinations made for reports received during 2002 through 2006 involving right, humpback, fin, sei, blue, minke, and Brydes whales along the eastern seaboard of the United States and adjacent Canadian Maritimes. A total of 469 unique large whale events were reported during the period, including carcasses (both beached and at sea) and live whales sighted at sea. We received 160 entanglement reports and 43 reports of ship strikes. We were able to confirm 145 unique entanglement, 43 ship strike, and 314 mortality events. Twenty-one (14%) of the entanglements and 27 (63%) of the ship strikes were fatal. Serious injury was sustained in 16 (11%) of the entanglement events and in 2 (5%) of the confirmed ship strikes. Twenty-three (16%) of the entanglements and 3 (7%) of the ship strike events did not have adequate documentation to determine if serious injury occurred. Seventy-eight (54%) of the entanglement events and 9 (21%) of the ship strike events were determined to have not caused serious injury or death. Of the confirmed mortalities, 249 (79%) lacked sufficient evidence to determine cause of death. Minke whales had the greatest number of entanglement mortalities (n=9); humpback whales had the highest number of serious injury events resulting from entanglements (n=9); and right whales had the greatest number of ship strike mortalities (n=10) and serious injuries (n=2) from ship strikes. Poor detection probabilities and inadequate documentation have likely reduced these tallies significantly. The true level of human impact to these stocks is unknown.


As part of the 1994 amendments to the Marine Mammal Protection Act (MMPA), the NOAA National Marine Fisheries Service (NMFS) was mandated to establish monitoring programs to obtain statistically reliable estimates of incidental mortality and serious injury of marine mammals taken during commercial fishing operations. The Agency was also charged with developing Take Reduction Plans (TRPs) to reduce commercial takes of strategic stocks of marine mammals below the Potential Biological Removal (PBR) levels specified in the TRPs within six months after plan implementation. The longer-term goal of all the TRPs is to reduce – within 5 years of implementation – commercial takes of marine mammals to insignificant levels approaching zero mortality and serious injury rates.

In April 1997, NMFS convened a Serious Injury Workshop to develop a consistent set of guidelines for determining what constitutes a serious injury (Angliss and DeMaster 1998). Although the workshop produced a set of recommendations, implementation of a national serious injury standard has not yet occurred.

Nonetheless, NOAA Fisheries staff and Scientific Review Group (SRG) members decided to take account of serious injuries in the annual marine mammal stock assessment reports (SAR). Subsequently, the Northeast Fisheries Science Center (NEFSC) implemented the Workshop’s large cetacean recommendations and since 1996 has annually determined serious injury and mortality of large whale stocks in Northwest Atlantic Ocean (Cole et al. 2006; Nelson et al. 2007).

The most current five years’ average rate of human-caused serious injury and mortality is reported for each species in the annual stock assessment report. This rate can be used as an index of the success of a recovery plan when compared to a population’s potential biological removal level (PBR). The PBR is the maximum number of animals, not including natural mortalities, that may be removed from a marine mammal stock while allowing that stock to reach or maintain its optimum sustainable population (Wade and Angliss 1997). The PBR is the product of the following factors:

1. the minimum population estimate of the stock;
2. one-half the maximum theoretical or estimated net productivity rate of the stock at a small population size; and
3. a recovery factor between 0.1 and 1.0.

This report presents the protocols and determinations for events involving right (Eubalaena glacialis), humpback (Megaptera novaeangliae), fin (Balaenoptera physalus), sei (Balaenoptera borealis), blue (Balaenoptera musculus), minke (Balaenoptera acutorostrata), and Brydes (Balaenoptera edeni) whale stocks along the U.S. eastern seaboard for the period 2002–2006.


Marine mammal strandings and human-induced interaction events were recorded and submitted to the NMFS Northeast Regional Office (NERO) and Southeast Regional Office (SERO) by members of the National Stranding Network, large whale disentanglement teams, the U.S. Coast Guard, and civilian sources. The Regional Offices identified and obtained all available information for each event (photos, necropsy reports, etc.) and placed these in a central folder for each event. Case files were also compiled for all individually identified whales with injuries. Several NEFSC and NERO staff members were involved in reviewing event records, confirming each event’s occurrence and the species involved, identifying duplicate records, and consolidating unique information from each source into a single record for each event. Information from additional sightings of a previously documented event was added to the original event record. If an identified whale was involved in a second interaction, a new event record was assigned. The NEFSC staff then reviewed each mortality event and assigned a cause of death following the confirmation criteria listed below. Each injury event was similarly examined for indications of cause, and identified as a serious injury if it was likely to lead to the whale’s death. One staff member (TVC) reviewed all determinations each year to ensure consistency in the application of determination criteria within and across years.

Event and Species Confirmation Criteria

Events and the species involved were considered confirmed if they met one of the following criteria:

  1. the event was observed by a trained marine mammal observer who was certain of the species or event;
  2. the event was observed by a trained member of the Disentanglement Network and the species or event was verified via interview by NMFS, disentanglement or stranding network staff;
  3. the report was accompanied by photographs or videotape of sufficient quality to verify the species or event;
  4. a fisherman reported a whale entangled in his/her gear or a shipper reported colliding with a whale; or
  5. gear was retrieved from a whale.

Events and the species involved were considered confirmed in the following less certain cases:

  1. the observer was a trained marine mammal observer and was fairly certain, but not positive, of the species or event;
  2. the observer was inexperienced, but was interviewed by trained staff and the account was descriptive enough that the species or event was probable but not certain; or
  3. the report was accompanied by poorer quality photographs or video, and staff reviewing this material assessed the event as probable but not certain.

Events or the species involved were considered unconfirmed if:

  1. the observer was inexperienced and no photographs or video were taken, and the observer’s account did not provide sufficient detail to identify the species or event occurrence;
  2. the observer was experienced, but did not see the whale long enough or in good enough conditions to state the species or event as being probable;
  3. the event was photographed or video taped, but staff reviewing the images could not identify species or the event’s occurrence; or
  4. a carcass was too decomposed to identify species or to show any indication of human interaction.
Human-Induced Mortality Determinations

Events were categorized as entanglement mortalities if both of the following indications were confirmed to be present during gross inspection or necropsy of the carcass:

  1. fishing line constricted any body part; and
  2. subdermal hemorrhaging or extensive necrosis was present at point of attachment.

Events were categorized as ship strike mortalities if one of the following indications was confirmed to be present on a carcass:

  1. large linear lacerations (anywhere on body, as opposed to just dorsally as in Kraus 1990);
  2. large areas of subdermal hemorrhaging, hematoma, or edema;
  3. extensive skeletal fracturing; or
  4. a code 2 (fresh dead) carcass was brought in on the bow of a ship.
Serious Injury Determinations

Events were categorized as entanglement serious injuries if one of the following indications was confirmed on a living whale:

  1. fishing line constricted any body part, or was likely to become constricting as the whale grew;
  2. it was uncertain if the line was constricting, but appendages near the entanglement’s point of attachment were discolored and likely compromised;
  3. the whale showed a marked decline in appearance following entanglement, including skin discoloration, lesions near the nares, fat loss, or increased cyamid loads;
  4. gear was ingested; or
  5. whale was anchored.

A whale was typically not considered seriously injured if all constricting lines were removed or shed.

Events were categorized as ship-strike serious injuries if, following the appearance of a linear laceration or large gouge, a living whale exhibited a marked decline in appearance, including skin discoloration, lesions near the nares, fat loss, or increased cyamid loads.

Injuries that impaired the whale’s locomotion or feeding were not considered serious injuries unless they were likely to be fatal in the foreseeable future. No forecasts were made as to how an entanglement or injury might increase the whale’s susceptibility to further injury (e.g., from additional entanglement or collisions with vessels).


A total of 469 events was reported during 2002 - 2006, involving both live and dead whales (Table 1). There were 160 reports of entanglement and 43 of ship strike. From these, we confirmed 145 entanglement events and 43 ship strike events. We were able to verify 314 mortalities, and determine that 21 mortalities were due to entanglements and 27 mortalities were the result of ship strikes. Entanglement was determined to have caused serious injury in 16 events, and 2 serious injury events were determined to have resulted from ship strike. Table 2 presents a summary of mortalities attributed to causes other than entanglement or ship strike, confirmed entanglement and ship strike events not resulting in serious injury or mortality, and confirmed events for which insufficient information was available for determination. The cause of death could not be established for 249 (79%) of the verified mortalities. There were 78 entanglement events which did not result in serious injury (this includes cases where the animal was disentangled or shed gear), and 23 which lacked sufficient evidence to determine if serious injury had occurred. Nine ship strike events occurred which did not result in serious injury, and three events lacked sufficient evidence to make a determination. Annual human-caused mortality and serious injury rates for 2002 - 2006 are presented for each large whale stock in Table 3. Tables 4 to 9 provide details of each confirmed serious injury or mortality record.

Over the five-year period, right whales had the highest proportion of entanglements and ship strikes relative to the number of reports for a species: of 54 reports involving right whales, 25 were confirmed entanglements and 16 were confirmed ship strikes (Table 1). There were 21 verified right whale mortalities, three due to entanglements, and ten due to ship strikes. Serious injury was documented for four entanglement events and two ship strikes involving right whales (details in Table 4).

Humpbacks were involved in 181 reported events (Table 1). Of these, 77 of the 86 reported entanglements could be confirmed, as could 9 of the 10 reported ship strikes. Humpbacks were the most commonly observed entangled whale species and the most commonly observed dead whale (101 confirmed mortalities). Entanglements accounted for six mortalities and nine serious injuries. Ship strikes were relatively uncommon, with only nine confirmed events, seven of which were fatal. Details are provided in Table 5. Contrary to previous determination reports, here we assumed all humpback events involved members of the Gulf of Maine stock unless a whale was confirmed to be from another stock. At the time of this writing, there was no available information to indicate an event did not involve a Gulf of Maine animal (see Table 3).

Fin whales had a low proportion of entanglements. Of 51 reported events, eight were entanglements (all confirmed); two of these were fatal and two resulted in serious injury. Nine ship strikes were reported, eight of which were confirmed, and six proved fatal. Details are provided in Table 6.

Only eight events were reported for sei whales. There was one report of entanglement, which was confirmed and resulted in serious injury. The remaining seven events were confirmed mortalities. Two of the mortalities were determined to have resulted from ship strikes (see Table 7).

Minke whales were involved in 98 reported events. Entanglements accounted for 31 of these reports, but only 27 could be confirmed (Table 1). Nine of the confirmed entanglement events were fatal, the highest percentage for any of the whale species. There were only two confirmed ship strike events, both of which resulted in mortality (Table 8).

Blue and Brydes whales had the lowest number of reporting events for all species. Blue whales appeared in only one reported event which was a confirmed entanglement in the St. Lawrence River,Canada . There was not sufficient information available to confirm if a serious injury occurred. Of the two reported events for Brydes whales, one was a confirmed entanglement which resulted in the death of the whale (See Table 9).

In 74 of the 469 large whale events reported during 2002 - 2006, positive species identification was not possible (Table 1). In nine events, the similarity in body shape and size between fin and sei whales prevented us from distinguishing which of these two species were involved. In another 15 events, the whales could only be identified as balaenopterids based on the presence of ventral pleats. The taxonomic identity of the whales involved in the remaining 50 events could not be assigned with any certainty. Entanglement was reported in seven of these cases, five of which were considered confirmed. Sixty of the 74 reported events involving unidentified whales were confirmed mortalities.


Differentiating causal injuries from pre-existing ones or post-mortem damage is problematic, but can be accomplished through examination of necropsy data. In our determinations, fishing line constrictions were considered circumstantial evidence of pre-mortem entanglement, as these constrictions were likely the result of force applied by an active animal. Vessel collisions frequently lack external evidence, and may not be detected unless a necropsy is conducted; necropsies frequently identified subdermal hemorrhaging or hematomas, indicating that blood was still circulating at the time of injury. Large lacerations were considered an indication of a pre-mortem vessel collision since only whales at depth would be exposed to the propellers of a ship; floating carcasses would be pushed aside by the ship’s bow wave (Knowlton et al. 1995).

Events involving constricting entanglements with evidence of the whale’s deteriorating health were considered confirmed serious injuries. A whale’s physiological response to tissue damage includes increased secretion of glucocorticoids, which suppresses lymphocytes, and if sustained (due to chronic destruction of tissue by gear) compromises the ability of an animal to fight other infections. External indications of poor health, including skin discoloration, lesions near the nares, fat loss, or increased cyamid loads are part of a cascade of immunological disorders. Removal of constricting gear was generally considered to prevent serious injury. Whales only loosely entangled in line typically did not have external indications of poor health; some whales carried loose wraps for years.

We recently were made aware of an additional concern for entanglements involving the mouth. Large whales likely require a hydrostatic oral seal to create negative pressure inside the mouth, keeping the mouth closed by suction. If an entanglement fouls the baleen and trails out of the mouth, or results in tissue damage that deforms the lips, this seal could be rendered ineffective, resulting in a substantial energetic drain as the whale is forced to hold the mouth closed muscularly. Unless the gear is removed, or the injury heals enough to allow formation of the oral seal, such entanglements are likely to be lethal (Lambertsen et al. 2005).

Over the five-year period, 249 of 314 confirmed mortalities (79%) lacked sufficient evidence to determine cause of death (Table 2). Carcasses floating at sea often cannot be examined sufficiently for either internal or external indications, and generate false negatives if they are not towed ashore and necropsied. Likewise, insufficient documentation precluded determination in 23 of 145 confirmed entanglement events (16%) and 3 of 43 ship strike events (7%).

However, our greatest concern remains the number of animals never observed. Humpback whale scar evidence suggests that only 3-10% of entanglements are witnessed and reported (Robbins and Mattila 2000, 2004). Thus, whales may succumb to entanglement before the event can be detected. Negatively buoyant species are less likely to be detected after death, and positively buoyant species, such as right whales, may become negatively buoyant if an injury precludes effective feeding for an extended period (Moore et al. 2004). The numbers in this report therefore represent the minimum values for human-caused serious injury and mortality to large whale stocks along the U.S. eastern seaboard.


We are especially grateful to the East Coast stranding and entanglement networks, whose members searched for and examined whales both live and dead. It is a difficult and smelly job that deserves special recognition. The United States Coast Guard was instrumental in conveying sightings reported by mariners, investigating carcasses at sea and assisting in disentanglement efforts. We are also grateful to the staff of the ProvincetownCenter for Coastal Studies and New England Aquarium, NOAA aerial survey teams, Wildlife Trust, the States of Florida/Georgia and many others for providing the sightings that have allowed this work to be conducted. Liz Pomfret-Wiley, Amy Whittingham Chase, Brenda Rone and Misty Niemeyer verified records. Members of the Atlantic Scientific Review Group have provided numerous useful comments on the protocols described here. We also thank the anonymous reviewers of earlier drafts of this report.


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Cole TVN, Hartley DL, Garron M. 2006. Mortality and serious injury determinations for baleen whale stocks along the eastern seaboard of the United States, 2000-2004. US Dep Commer, Northeast Fish Sci Cent Ref Doc. 06-04; 18 p. Available at:

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Moore MJ, Knowlton AR, Kraus SD, McLellan WA, Bonde RK. 2004. Morphometry, gross morphology and available histopathology in North Atlantic right whale (Eubalaena glacialis) mortalities (1970–2002). J Cetacean Res Manage. 6(3):199-214.

Robbins J, Mattila DK. 2000. Monitoring entanglement scars on the caudal peduncle of Gulf of Maine humpback whales: 1997-1999. Center for Coastal Studies. Order no. 40ENNF900253; 24 p.

Robbins J, Mattila DK. 2004. Estimating humpback whale (Megaptera novaeangliae) entanglement rates on the basis of scar evidence. Report to the National Marine Fisheries Service. Order no. 43ENNF030121; 22 p.

Wade PR, Angliss RP. 1997. Guidelines for assessing marine mammal stocks: Report of the GAMMS Workshop, April 3-5, 1996, Seattle, Washington. US Dep Commer,NOAA Tech Memo NMFS OPR 12; 93 p.
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