Susan Gwynn Barco, Margaret Cook Lynott, William Mark Swingle

Susan Gwynn Barco, Margaret Cook Lynott, William Mark Swingle
Virginia Aquarium & Marine Science Center, 717 General Booth Blvd., Virginia Beach, Virginia, 23451‐1855, USA
Author email: [email protected]
From 2000‐2010, 716 bottlenose dolphin (Tursiops truncatus) strandings were reported in Virginia. We examined carcasses for signs of human/fishery interaction (HI) and made a determination in 296 (41%). 183 (62%) had HI. We saw no HI in 113. We could not determine HI in 420 cases. When a dolphin scored YES for HI, we examined marks to determine gear type for ld
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fishery interactions. We distinguished monofilament from multifilament (twisted twine) mesh in 115 cases. In Virginia, twisted twine mesh is consistent with pound net gear (PNG) and monofilament mesh is consistent with gill net gear (GNG). Ninety (78%) cases were consistent with PNG and >80% occurred near pound nets in Virginia Beach, VA. In 2010, Virginia began requiring an alternative leader with 2/3 less mesh than traditional leaders in this area. The 10 year mean of Tursiops
strandings with marks consistent with PNG prior to 2010=6.8 (±2.3), median=6 Although there has only been one year of stranding data, 2010 strandings consistent with PNG=4. For marks consistent with GNG, the 10 year mean=2.3 (±3.1), median=1 and 2010 strandings=1. Seasonally we divided strandings consistent with PNG and GNG into summer (July‐
September) and non‐summer. In Virginia PNG is usually active from March‐October and GNG is used nearly year‐round with less effort in summer. Strandings consistent with GNG tended to occur in non‐summer (n=17; 71%), while strandings
consistent with PNG tended to occur in summer (n=65; 72%). With new stock assessments for coastal Tursiops suggesting that as many as four stocks occur in Virginia, assigning takes to a specific stock will be problematic. Understanding the effect
and seasonality of coastal fisheries such as the pound net and gill net fisheries on Tursiops may depend largely on continued investigation into stranding data. CBD 420
MONO
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HI 183
TWIST
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No HI 113
In the images above we show pound net gear and marks consistent with pound net gear: A‐pound net interaction where a dead dolphin was removed from a pound net; B‐
marks on the head of a bottlenose dolphin consistent with twisted twine which is the twine type used in pound nets; and C‐twisted twine. In the charts above, the pie chart on the left indicates the number of stranded Tursiops that scored positive for human interaction (HI), negative for human interaction (No HI) and where the likelihood of human interaction could not be determined (CBD). Note that the majority of carcasses (58%) did not have a definitive HI determination due primarily to carcass condition. Where a determination could be made (HI or No HI) nearly two‐thirds (63%) were HI, most of which included fishery interaction.
In the chart on the right, of the 183 animals that scored positive for HI, we could determine that net gear of a specific twine type was involved in the interaction in 115 cases (62%). The MONO category includes animals that stranded with gillnet gear attached or with marks consistent with monofilament net (22%), and the TWIST category (78%) included animals removed from pound nets dead or with marks consistent with twisted twine net.
In the chart below, we have used a radial graph to represent the calendar year and have collapsed all strandings onto one year in order to show the seasonality of Tursiops
HI strandings associated with pound nets and twisted twine net (TWIST). Note that these strandings are largely responsible for the summer seasonality of HI in Tursiops.
Seasonality of Tursiops net gear interactions with twisted twine/pound net (TWIST)
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In the images above we show gill net gear and marks consistent with gill net gear: A‐a stranded dolphin with gill net gear attached; B‐marks on the flipper of a bottlenose dolphin consistent with monofilament net which is the twine type typically used in gill nets; and C‐monofilament twine
In the chart below, we indicate the seasonality of those HI strandings involving gill net gear and monofilament. These stranding tend to occur in the spring and fall and happen less frequently than pound net interactions.
Seasonality of Tursiops net gear interactions with monofilament twine/gill net (MONO)
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In the chart below, we have used a radial graph to represent the calendar year and have collapsed strandings onto one year in order to show the seasonality of Tursiops strandings in general (green) and of animals that scored positive for human interaction (HI; yellow). Note here that Tursiops strandings in VA peak in early May and remain steady through the spring and summer, but that dolphins positive for HI are most common in the summer. F
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Seasonality of all Tursiops strandings (green) with those that scored positive for human interaction (HI; yellow)
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All strandings
All HI
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Number of strandings
In 2005 some pound nets on Virginia’s eastern shore were restricted unless modified leaders intended to reduce sea turtle by‐catch were used. Pound net HI with bottlenose dolphins continued, however, and the highest density of strandings occurred near the mouth of Chesapeake Bay in an area where pound nets were not required to use modified leaders (D’Eri et al. 2009). Because of by‐catch, the Aquarium worked with a fisherman in the area to test the sea turtle leader (Schaffler et al. 2011) and these leaders were adopted in the area in 2010. In the chart to the right, TWIST HI in Tursiops appears to have decreased since these leaders were required in 2010 in the area of highest stranding density. 14
Gear mods partially implemented
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required in all areas
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D’Eri, Linda R., Susan G. Barco, W. Mark Swingle and Gwen G. Lockhart. 2009. Bottlenose dolphin (Tursiops truncatus) fishery interactions: solutions to pound net entanglements in Virginia. Presented to the Northeast Regional Stranding meeting. Boston, MA. April 3‐7.
Schaffler, Jason, J. Susan G. Barco, Linda R. D’Eri, W. Mark Swingle and Cynthia M. Jones. 2011. Do Alternative Leaders Affect Fish Catch in Pound Nets at the Mouth of Chesapeake Bay? North American Journal of Fisheries Management 00:1–8, 2011
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2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
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We would like to thank the Virginia Aquarium & Marine Science Center Foundation and the Stranding Response Team staff and volunteers for support and assistance over the years. Stranding response in Virginia has been partially supported by the John H. Prescott Marine Mammal Grant Program since 2002, and, without that support, the quality and consistency of our marine mammal stranding data, investigations and presentations would be significantly lower. Finally we would like to thank Bill McLellan, Denise Boyd and Katie Touhey Moore for their assistance, ideas and discussions about human interaction and stranded marine mammals. Pound net gear modification research was conducted under a contract (08‐DMM‐03/2005‐2042‐04) from North Carolina Sea Grant in 2008.