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The 1990 Tursiops truncatus (Common Bottlenose Dolphin) Mass Die-off in East Matagorda Bay, Texas: New Insight into a Cold Case
Errol I. Ronje, Heidi R. Whitehead, and Keith D. Mullin

Southeastern Naturalist, Volume 17, Issue 3 (2018): 411–422

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Southeastern Naturalist 411 E.I. Ronje, H.R. Whitehead, and K.D. Mullin 22001188 SOUTHEASTERN NATURALIST 1V7o(3l.) :1471,1 N–4o2. 23 The 1990 Tursiops truncatus (Common Bottlenose Dolphin) Mass Die-off in East Matagorda Bay, Texas: New Insight into a Cold Case Errol I. Ronje1,*, Heidi R. Whitehead2, and Keith D. Mullin1 Abstract - On 20 January 1990, twenty-three Tursiops truncatus (Common Bottlenose Dolphin) carcasses were found scattered around the interior shoreline of East Matagorda Bay, TX. Few accounts exist to document the presence of live or dead Common Bottlenose Dolphins inside the boundaries of East Matagorda Bay before or after the die-off. We conducted a review of areal data for East Matagorda Bay and the original investigation of the January 1990 mass die-off. Information we examined included the history of natural and anthropogenic changes to the area, dolphin stranding records, small-boat visual surveys, and dolphin dorsal-fin photographic identification. Natural events preceding the discovery of the dolphin carcasses were likely factors in the mortalities; however, the timing of engineering projects that modified access points between the bay and the Gulf of Mexico may be an additional factor that contributed to the cause of this unusual mortality event. Introduction In the northern Gulf of Mexico (southern Texas to southern Florida), 344 Tursiops truncatus (Montagu) (Common Bottlenose Dolphin, hereafter Dolphin) died in a Gulf of Mexico (GoMx)-wide unusual mortality event (UME) from January to May 1990 (Litz et al. 2014). Though a definitive cause for the overall UME was not found, a morbillivirus pathogen may have been a causal factor in the majority of cases (Litz et al. 2014). An early incident of the UME was a highly localized die-off in East Matagorda Bay, TX (Fig. 1), for which it is believed other factors were responsible. On 20 January 1990, twenty-three carcasses of advanced decomposition were found scattered around the interior shoreline of East Matagorda Bay, TX. The Dolphin group’s composition represented a demographic cross-section comprising 11 males (6 adults, 5 juveniles), and 12 females (7 adults, 4 of whom were pregnant, and 5 juveniles) (Miller 1992). Following the die-off, Miller (1992) conducted an investigation into possible causal factors that included unusually cold weather that froze the surface waters of the bay for 2–3 d, an abnormally low tide that may have restricted the Dolphins’ ability to leave the bay for warmer waters, and a freeze-related fish kill that likely limited available prey, resulting in malnutrition for what was presumed to be a resident Dolphin population (Miller 1992). However, few accounts exist to document the presence of live or dead Dolphins inside the boundaries of East Matagorda Bay before the 1National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Science Center, Mississippi Laboratories, Pascagoula, MS. 2Texas Marine Mammal Stranding Network, Galveston, TX 77551. *Corresponding author - Manuscript Editor: Barbara E. Curry Southeastern Naturalist E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 412 die-off, and it is not clear if the Dolphins discovered in January 1990 were longterm inhabitants of the bay. A review of the sceintific literature indicated that East Matagorda Bay had not been studied to determine the presence or absence of Dolphins, but studies have indicated the much larger Matagorda Bay to the southwest is long-term Dolphin habitat (Barham et al. 1980, Gruber 1981, Lynn and Würsig 2002, Würsig and Lynn 1996). It is possible that East Matagorda Bay did not support a resident group of Dolphins prior to January 1990, and more information is needed to evaluate the Dolphin’s historical and current use of the bay. To that end, we conducted a review of areal data for East Matagorda Bay and the January 1990 Dolphin die-off, including the history of natural and anthropogenic changes to the area as documented in the literature and in government reports, aerial photography, and marine-mammal–stranding records. We also conducted small-boat visual surveys to assess the current presence or absence of Dolphins in and explore the status of waterway access to East Matagorda Bay. Field-site Description Background The GoMx Texas shoreline measures some 600 km (White and Calnan 1990) with numerous industrial ports that feature deep-draft channels dredged to allow for the safe transit of ships between each port and the GoMx. Prominent among these channels is the Gulf Intracoastal Waterway (GIWW), a dredged commercial-shipping artery along the GoMx coast of the southeastern US, and in Texas, the GIWW spans the coast from Brownsville to Sabine Lake (USACE 1992). Other engineered passes, or “cuts” as the smaller passes are colloquially known, are dredged to improve seawater exchange with estuarine communities for the benefit of aquaculture or recreational fishing (e.g., Rollover Pass and Packery Channel, TX; Kraus 2007). Although there are several different methods of dredging (Todd et al. 2015), the processes typically involve the mechanical removal of marine aggregate, followed by a deposition of the dredge spoils in another location. Dolphins inhabit the state’s bays, estuaries, and coastal waters (Phillips and Rosel 2014) and at times may be found concentrated in dredged channels or passes (Fertl 1994, Gruber 1981, Henderson and Würsig 2007, Lynn and Würsig 2002, Maze and Würsig 1999, Moreno 2005, Shane 1980). Dead, sick, or injured Dolphins are sometimes found stranded on shorelines (Colbert et al. 1999, Hansen 1992, Worthy 1998), prompting a response from the Texas Marine Mammal Stranding Network (TMMSN). East Matagorda Bay East Matagorda Bay (Fig. 1) is a shallow lagoon (average depth ~1 m) of moderate salinity (20–24 ppt) comprising approximately 155 km2 (Cifuentes et al. 2006, Craig et al. 1989, Froeschke et al. 2010, Gardner et al. 2006, Kraus and Militello 1999, Palmer et al. 2011). Oriented southwest to northeast along the northerncentral Texas Coastal Bend, East Matagorda Bay is surrounded by engineered waterways including the Colorado River Navigation Channel (CRNC), the GIWW, and a short, narrow channel—Mitchell’s Cut (Gardner et al. 2006, King and Prickett Southeastern Naturalist 413 E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 1998, USACE 1992, USACE 2017, USGPO 2017). East Matagorda Bay was separated from Matagorda Bay after a massive log jam was cleared from the Colorado River mouth near the town of Matagorda in 1929 (Fig. 2; King and Prickett 1998). The clearing of the log jam resulted in the rapid development of the Colorado River delta in the 1930s, closing off the eastern section of Matagorda Bay and resulting in one of the most isolated bodies of water on the Texas GoMx coast in terms of distance to inlets where seawater exchange can occur (Froeschke et al. 2010, Gardner et al. 2006, Wadsworth 1966, White and Calnan 1990). Navigational locks—23-m– wide steel doors spanning the GIWW channel—were constructed during the 1940s where the Colorado and Brazos rivers meet the GIWW. The locks control sediment movement and excessive tidal action; they are open on-demand for commercial and emergency-vessel traffic but are typically restrictive to the passage of recreational vessels (USACE 2017, USGPO 2017). In the early 1990s, the Bypass Channel was constructed around the east Colorado River Lock to allow direct transit between the CRNC and GIWW, but prior to its construction, all GIWW traffic transiting northeast was routed through the navigational locks at the Colorado River (McCollum 2000). East Matagorda Bay lacks a natural seawater pass directly into the bay, although a natural ephemeral pass, Brown Cedar Cut, has previously been a conduit for seawater exchange (Mason and Sorensen 1972, McGowen et al. 1979, Miller 1992, Munro 1965, USFWS 1988). Figure 1. East Matagorda Bay landmarks, Common Bottlenose Dolphin sightings, and survey tracks. Surveys were conducted on 30 July 2016 and 27–29 June 2017. Landmarks: 1. Colorado River Navigation Channel jetty-impoundment basin, 2. Colorado River Diversionary Canal, 3. Gulf Intracoastal Waterway west lock, 4. Gulf Intracoastal Waterway east lock and Bypass Channel, 5. Old Gulf Cut, 6. Big Boggy Cut, 7. Brown Cedar Cut (closed), 8. Mitchell’s Cut, and 9. McCabe Cut (closed). Southeastern Naturalist E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 414 Methods Data review Scientific literature. We reviewed literature relevant to the greater Matagorda Bay system to establish a timeline of major engineering projects and natural events impacting the waterways near East Matagorda Bay within 10 y prior to the January 1990 East Matagorda Bay die-off and to determine the extent to which GoMx access points and adjacent waterways were modified (King and Prickett 1998, Stauble et al. 1994, USACE 1988, USACE 1992). We also examined records from the US Army Core of Engineers (USACE), specifically all documentation pertaining to the USACE permit #18399 (a permit granted for the construction of Mitchell’s Cut, also known as Caney Creek Fork Cut), obtained by a Freedom of Information Act (FOIA) request to USACE, and aerial photographs obtained by the US Geological Survey (USGS) National Aerial Photography Program (NAPP) of East Matagorda Bay collected 10 December 1989. Records of marine mammal strandings. The TMMSN maintains archives of all marine-mammal strandings recorded in Texas since its inception in 1980, and since 1990 has transmitted all stranding data to the NMFS Marine-Mammal Health and Stranding Response database (MMHSRP). Stranded animals recovered by the TMMSN are assigned field-identification numbers that identify the region in which the animal was found. The TMMSN divides the state of Texas into 6 response regions; Matagorda Bay and East Matagorda Bay are included in the region designated as Port O’Connor (PO). We extracted, and cross-checked for consistency, stranding records from the TMMSN (1980–2017) and the MMHSRP (1990–2017) corresponding to the PO region. We plotted all strandings designated Figure 2. The town of Matagorda and Matagorda Bay in 1915, before the formation of the Colorado River delta that resulted in East Matagorda Bay as a separate body of water. Image from NOAA's Office of Coast Survey Historical Map and Chart Collection, http:// Southeastern Naturalist 415 E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 as PO in ArcGIS (Esri ArcMap®10.4, Redlands, CA) to quantify the number of strandings in Matagorda and Calhoun counties and verify the physical locations described in each stranding record. Visual surveys We conducted visual surveys during July 2016 and June 2017 in East Matagorda Bay, its connecting waterways, and adjacent GoMx waters (Fig. 1). Surveys covered the central portion of the bay, the interior perimeter (~300–500 m from shore or as depth allowed), and connecting waterways including Caney Creek, Mitchell’s Cut, the GIWW, the CRNC and the Bypass Channel, and the Colorado River Diversionary Canal. We surveyed GoMx waters adjacent to East Matagorda Bay ~500 m from the beach along the length of Matagorda Peninsula during 1 survey. We modeled the surveys after NMFS Dolphin visual surveys designed to collect photo-identification (photo-ID) capture–mark–recapture (CMR) data (Melancon et al. 2011). Photo-ID CMR is a decades-old technique used to collect data for population analyses that photographically captures the distinct dorsal-fin markings of individuals in a population of delphinids (Shane 1977, Würsig and Würsig 1977), yielding data that can be used to support population assessments (Rosel et al. 2011). Results Data review Scientfic literature. Resolutions to the pre-dredging objections from federalgovernment agencies (e.g., NMFS) and non-governmental organizations concerning the effects of the construction on terrestrial and marine species were found during the review of permit application #18399 to the USACE and the subsequent environmental review for the construction of Mitchell’s Cut; however, Dolphins were not listed as potentially inhabiting the area (USACE 1988). Based on the literature review, a timeline of anthropogenic modifications to East Matagorda Bay during 1983–1990 (Fig. 3; Stauble et al. 1994, USACE 1992) indicates that ~9 km to the northeast of Brown Cedar Cut, another waterway known as McCabe Cut was mechanically opened in 1983 as a flood-relief measure for the nearby community of Sargent, located on the banks of Caney Creek, 8 km north of the GIWW. Mc- Cabe Cut was initially designed with a bottom width of approximately 12 m and a depth of 0.6 m at mean low tide (USACE 1992); however, it was mechanically filled in March 1989 to resolve navigational hazards resulting from a cross current that developed after the cut grew in size due to the erosional effects of the GoMx (USACE 1988). As an alternative measure to alleviate flooding in Sargent, dredging operations were completed in May 1989 on Mitchell’s Cut, a 1800-m–long channel dredged to a depth of 2.4 m, approximately 4 km to the southwest of Mc- Cabe Cut, closer to East Matagorda Bay (Rosati III et al. 2013, USACE 1988). In the months leading up to and temporally overlapping with the UME, 2 dredging projects were conducted in waterways adjacent to the bay. Approximately 135,215 m3 of sediment were removed from the GIWW opposite the former McCabe Cut and deposited on the GoMx shore at Sargent Beach during GIWW maintenance Southeastern Naturalist E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 416 dredging from September 1989 to January 1990 (USACE 1992). While the maintenance dredging of the GIWW occurred, another dredging operation, undertaken from September 1989 to March 1990, removed 1,318,038 m3 of sediment from the jetty-impoundment basin and entrance channel of the CRNC where it meets the GoMx (King and Prickett 1998). Aerial photographs acquired 10 December 1989 (NAPP 2017) confirm no bypass channel existed to circumvent the east navigational lock at the Colorado River. The photos indicate that McCabe Cut and Brown Cedar Cut were closed, and Mitchell’s Cut and the mouth of the CRNC appeared open, but with apparent silting (breaking water on presumed sedimentation) present at the opening of both passes. Dredging operations were not visible in the aerial photos for 10 December 1989. Marine mammal stranding records. The stranding archives (TMMSN and MMHSRP) for the state of Texas from 1980 to 2017 indicated that 123 Dolphins stranded on the interior shores (e.g., excluding GoMx beach strandings) of Matagorda Bay and its associated waters (e.g., Espiritu Santo Bay) in Matagorda and Calhoun counties. The East Matagorda Bay die-off of January 1990 was the only record of dead stranded Dolphins reported in the interior of East Matagorda Bay, although 2 response cases occurred for live Dolphins in the vicinity. In the first case, a 264-cm male Dolphin (TMMSN field # PO331) was found stranded alive in the GIWW near Old Gulf Cut (Fig. 1). In the second case, a live Dolphin (TMMSN field # PO528) was found swimming and foraging “out-of-habitat” (Rosel and Watts 2007) in a road-side ditch (depth ≤ 0.75 m) parallel to the CRNC near the southwest corner of East Matagorda Bay on 13 April 2017 (Whitehead and Ronje 2017). Figure 3. Timeline of events in East Matagorda Bay (EMB) 1983–1990 related to topographical modifications and Common Bottlenose Dolphin observations referenced in Miller (1992). Southeastern Naturalist 417 E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 Visual surveys Our small-boat surveys consisted of a total of 29.0 survey hours, and, over the course of 666.0 km surveyed, we observed 7 Dolphin groups (Fig. 1). We saw no Dolphins inside East Matagorda Bay proper during the surveys but observed them outside of the bay in the channels of the GIWW and CRNC, as well as in the GoMx. On 30 July 2016, we observed a single Dolphin foraging behind a trawl vessel on the GoMx coast along the Matagorda Peninsula, 1 group of ~12 Dolphins near the mouth of the CRNC in the GoMx, and another single Dolphin ~8 km upstream of the river mouth in the CRNC. During the 27–29 June 2017 surveys, we observed a total of 6 individual Dolphins foraging in the CRNC or GIWW (3 presumed mother/calf pairs), as determined by the photo-ID analysis. During the visual surveys, we noted few options for Dolphins to enter East Matagorda Bay. Brown Cedar Cut was closed, and Dolphins traveling in the GIWW from Matagorda Bay or the Brazos River mouth (~ 30 km northeast of Mitchell’s Cut) would need to contend with navigational locks. Only 2 opportunities appear currently plausible for Dolphins attempting to access the GIWW and the bay from the GoMx: the CRNC via the Bypass Channel, or Mitchell’s Cut. Mitchell’s Cut was considerably turbulent during some parts of our survey, and silting of the mouth where it meets the GoMx was evident. Along the northern side of the bay, we observed 2 potential access points into the bay from the GIWW (Old Gulf Cut and Big Boggy Cut; Fig. 1). Discussion Miller (1992) suggested that, during January 1990, Dolphins in East Matagorda Bay likely encountered a confluence of unfortunate events—an extreme low-tide event and a hard freeze of the bay, complicated by a fish kill that may have deprived the dolphins of prey when it was most needed. Tidal fluctuations are known to occasionally cause otherwise healthy Dolphins to inadvertently strand on mud flats or marshes (Gunter 1941, Sharp et al. 2016, Wiley et al. 2001). Although the tide was reported to be unusually low in December 1989 just before and during the freeze, ≥3 Dolphins were seen inside the bay following (possibly attempting to forage) behind a trawling vessel on 3 January 1990 (Miller 1992). It is not known if the Dolphins following the trawler were part of the group in the die-off, but if so, a trawling vessel in the bay may indicate a route of sufficient de pth was available to exit the bay into the GIWW before they died (e.g., through Big Boggy Cut). One dead Dolphin (TMMSN field # PO110) reported in Miller (1992) was found outside of the bay in the GIWW near the entrance of Big Boggy Cut (Fig 1.) The December 1989 freeze event froze East Matagorda Bay to a thickness of ~5 cm for 2–3 days resulting in an estimated fish kill of 2.7 million fish, the most significant on the Texas coast during that time (McEachron et al. 1994). However, previous freezes had occurred in East Matagorda Bay. The most severe freeze in Texas recorded history occurred 6 y prior, in December 1983, when the water temperature dropped 15 °C within 10 d and stayed between 0 °C and 5 °C for ~7 d; a freeze of shorter duration occurred in February 1989 (McEachron et al. 1994). The Texas Parks and Wildlife Department conducted extensive fish-kill surveys after Southeastern Naturalist E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 418 the December 1983 and February 1989 freezes in East Matagorda Bay (McEachron et al. 1994), but we found no stranding reports corresponding to either event. The lack of Dolphin-stranding reports from 1983 could be due to the TMMSN having only been established in 1980; the organization experienced a period of increasing public awareness after its inception. However, the TMMSN stranding records indicate government agencies (e.g., US Fish and Wildlife Service) were reporting strandings to the TMMSN from the Matagorda Bay area as early as 1981, and by 1985 reporting from the public and state agencies state-wide became consistent (Hansen 1992). Active dredging work has prompted negative short-term behavioral responses by marine mammals inhabiting the affected areas (Pirotta et al. 2013), but population studies of Dolphins along the Texas coast indicate that dredged channels or passes such as Sabine Pass (Ronje et al. 2017), Bolivar Roads (Mullin et al. 1990), Galveston Ship Channel (Fertl 1994), San Luis Pass (Henderson and Würsig 2007, Maze and Würsig 1999), Pass Cavallo (Gruber 1981), and Aransas Pass (Barham et al. 1980, Shane 1990) are among waters with the highest concentration of Dolphins in their respective bay systems. Deep passes or canals may act as bottlenecks that concentrate fish traveling between estuarine and marine habitats; these topographical features may have characteristics that boost species diversity and create foraging advantages (Allen et al. 2001, Anderwald et al. 2013, Maze and Würsig 1999, Shane 1990, Todd et al. 2015, Wilson et al. 1997). If dredged channels are an attractant for Dolphins, it is possible that Dolphins in nearby coastal waters explored Mitchell’s Cut after the dredging was completed in May 1989, resulting in access into East Matagorda Bay where few prior options existed. It is not certain that the Dolphins in the 1990 die-off were uncommon visitors to the bay before the cut was dredged, but the few stranding reports before and after the January 1990 Dolphin die-off and the results of our surveys here suggest East Matagorda Bay may not be typical habitat for Dolphins. Mitchell’s Cut may no longer serve as a reliable means for East Matagorda Bay ingress. Aerial photos indicate the mouth of the cut has a tendency to accumulate silt, and it may not be a consistent access point for Dolphins, as it is not maintained. In contrast, studies indicate that neighboring Matagorda Bay supports a Dolphin population in all seasons (Barham et al. 1980, Gruber 1981, Lynn and Würsig 2002, Würsig and Lynn 1996). The closure of McCabe Cut (March 1989) and the completion of Mitchell’s Cut (May 1989) may have been factors in the Dolphin mortality event in East Matagorda Bay of January 1990. Dolphins in the GoMx may have gained access through a new opening (Mitchell’s Cut) and encountered inhospitable natural conditions once inside the bay (freeze, low tide, fish kill). If a Dolphin group inhabiting East Matagorda Bay had attempted to leave for GoMx waters before the die-off, it is possible they were discouraged by the closure of their previous access point (McCabe Cut) and the multiple engineering projects. Given the navigational locks in place at the Brazos and Colorado rivers, and the lack of a bypass channel around the east Colorado River Lock during 1989–1990, it seems unlikely that Dolphins were transiting to or from the GoMx by means of these passages. In the event they were Southeastern Naturalist 419 E.I. Ronje, H.R. Whitehead, and K.D. Mullin 2018 Vol. 17, No. 3 doing so, the mouth of the CRNC and the GIWW east of the bay might have been disturbed by active dredging operations during the winter of 1989–1990. Visual surveys for dolphins in East Matagorda Bay and its connecting waterways should be conducted in association with dredging activities to safeguard the dolphins inhabiting the surrounding area. Our review may be useful as cautionary insight for future coastal-engineering projects that create new access to waters adjacent to Dolphin habitat or alter conditions to restrict or eliminate existing access points. Acknowledgments This manuscript was improved by helpful comments from an anonymous internal reviewer (NMFS), J. Reneker (NMFS), and J. Struve (University of Florida). Thanks to Elizabeth Stratton (NMFS) for assistance in obtaining MMHSRP stranding records and the TMMSN for assistance with stranding response and data collection. 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