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Evidence of Successful Spawning and Other Life-History Aspects of Alosa sapidissima (American Shad) in the Penobscot River and Estuary
Christine A. Lipsky, Rory Saunders, and Justin R. Stevens

Northeastern Naturalist, Volume 23, Issue 3 (2016): 367–377

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Northeastern Naturalist Vol. 23, No. 3 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 367 2016 NORTHEASTERN NATURALIST 23(3):367–377 Evidence of Successful Spawning and Other Life-History Aspects of Alosa sapidissima (American Shad) in the Penobscot River and Estuary Christine A. Lipsky1,*, Rory Saunders2, and Justin R. Stevens3 Abstract - Diadromous fish populations in Maine are near historically low levels. In the Penobscot River, ME, annual runs of Alosa sapidissima (American Shad) numbered in the millions prior to a collapse in abundance in the late 19th century. Today, the vast majority of historical American Shad spawning habitat is inaccessible to the fish; thus, there is uncertainty in terms of origin of the few extant American Shad that remain in the Penobscot. We used several types of sampling gear in the lower Penobscot River and Penobscot estuary as part of a community survey that documented the presence of juvenile American Shad throughout the estuary from July through August 2012. Our surveys indicated the presence of premetamorphic American Shad upstream of a salinity barrier, and therefore we conclude that there is a population of American Shad successfully spawning in the Penobscot River. Such evidence of a local stock is vitally important as managers weigh restoration options, such as stocking with donor stocks, enhancement of existing stocks, or natural recolonization. Introduction Alosa sapidissima (Wilson) (American Shad) are an important sport fish, food fish, and prey item. Unfortunately, contemporary abundance levels and distribution of American Shad are greatly reduced compared to historic levels (Limburg and Waldman 2009). In many instances, dams and other fish passage impediments block access to otherwise suitable spawning and rearing habitat (e.g., Sprankle 2005), thus these impediments remain substantial hurdles to restoring American Shad populations (Haro and Castro-Santos 2012). Considerable restoration efforts are currently underway to reverse American Shad declines, including substantial efforts in Maine such as the removal of the Edwards Dam on the Kennebec River, the Penobscot River Restoration Project (PRRP), and many smaller-scale efforts throughout the state. The recently completed PRRP involved the removal of 2 mainstem dams and the decommissioning and bypass of a third dam (Day 2006, Opperman et al. 2011). This project improved access to thousands of kilometers of rearing habitat for many diadromous species (Trinko Lake et al. 2012). If existing fishways at remaining dams function properly, American Shad will have access to 93% of their historic habitat (Trinko Lake et al. 2012). 1NOAA’s National Marine Fisheries Service, Northeast Fisheries Science Center, Maine Field Station, 17 Godfrey Drive, Suite 1, Orono, ME 04473. 2NOAA’s National Marine Fisheries Service, Greater Atlantic Regional Fisheries Office, Maine Field Station, 17 Godfrey Drive, Suite 1, Orono, ME 04473. 3Integrated Statistics, NOAA’s National Marine Fisheries Service, Greater Atlantic Regional Fisheries Office, Maine Field Station, 17 Godfrey Drive, Suite 1, Orono, ME 04473. *Corresponding author - Manuscript Editor: Jeremy Pritt Northeastern Naturalist 368 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 Vol. 23, No. 3 A well-recognized starting point in the rational management of sea-run fish involves the development of geographically explicit information on stock size, stock structure, and spawning locations. At the state level, Maine’s Department of Marine Resources (MDMR) has recently summarized existing river-specific information for extant American Shad populations in 4 of Maine’s largest rivers (MDMR 2014). The Maine Department of Inland Fisheries and Wildlife (MDIFW) and MDMR developed a multi-species management plan specific to the Penobscot River that set forth both strategic (MDMR and MDIFW 2008) and operational (MDMR and MDIFW 2009) goals for sea-run fish restoration. Unfortunately, little information has been available for American Shad in the Penobscot River until very recently. In fact, MDMR and MDIFW (2009) recently concluded that “no specific information is known about the stock structure, size, or spawning locations of American Shad in the Penobscot River or tributaries except that a remnant population exists.” Although no quantitative estimates were available at the time, MDMR and MDIFW (2009) estimated less than 1000 adult American Shad returned annually prior to the implementation of the PRRP. Recent evaluations by Grote et al. (2014a, b) revealed that American Shad are found in the lower Penobscot River in numbers greater than previously known, although no quantitative adult or juvenile population estimates are yet available. Additionally, the age structure, general migration timing, and habitat use of American Shad in the lower Penobscot River and estuary are not well understood (MDMR and MDIFW 2008, 2009). American Shad spawn in fresh water, and their larvae are not tolerant of salinity levels greater than 30 ppt until they undergo metamorphosis (between 26 and 45 d post-hatch; Jia et al. 2009, Zydlewski and McCormick 1997), when they transform into juveniles. This progression usually occurs when they are between 25 and 40 mm total length (Crecco et al. 1983, Greene et al. 2009, Liem 1924). In addition, premetamorphic American Shad were shown by Jia et al. (2009) to experience 50% mortality when subjected to salinity levels of 20 ppt for 27 d. Indeed, Limburg and Ross (1995) postulated that salinity could serve as a barrier that prevents premetamorphic American Shad from successfully emigrating from their natal rivers. Prior to the removal of the Veazie Dam (rkm 47.50) in 2013 and Great Works Dam (rkm 59.65) in 2012, Grote et al. (2014a) estimated that there were only 15 km of spawning habitat available to American Shad in the Penobscot River. Given the lack of targeted surveys for American Shad, coupled with the limited amount of spawning habitat, the extent of American Shad spawning in the Penobscot River remains an unanswered question. The objective of this study was to collect premetamorphic American Shad in conjunction with salinity data to determine if spawning is occurring in the Penobscot River, in order to demonstrate the presence of a local stock. This information is critical to managers as restoration proceeds and will help guide future management decisions. Field-Site Description The Penobscot River is the second largest river in New England and the largest watershed that lies entirely within the state of Maine, encompassing approximately Northeastern Naturalist Vol. 23, No. 3 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 369 22,000 km2 (Fig. 1). The Penobscot River is home to 12 native diadromous fish species (Saunders et al. 2006) and is widely viewed as an excellent opportunity for sea-run fish restoration at the national level (Martin and Apse 2011). Figure 1. Map of Penobscot River and estuary. Black rectangles represent sites of dams, and crosses represent former dam locations. Inset A is the regional location of the study area, and inset B depicts the sampling locations used in the study. Gray diamonds indicate seine sites, filled circles are 1-m fyke sites, open circles are 2-m fyke sites, and wavy lines represent trawl tow locations. Northeastern Naturalist 370 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 Vol. 23, No. 3 Our study site spans much of the Penobscot estuary (Fig. 1), which extends roughly 100 km from the head of tide near Bangor (44°48'4"N, 68°46'41"W), to Owls Head (near Rockland, ME; 44°4'56"N, 69°3'26"W; NEI 1985). The Penobscot estuary is a drowned-river estuary with a complex mixing regime that varies with freshwater flow, tidal height, temperature, and salinity (Haefner 1967). The mean depth of the Penobscot estuary is roughly 23.5 m (NEI 1985). Methods We conducted fish surveys throughout the Penobscot estuary between 10 July and 23 August 2012 using beach seines (n = 32), 1-m fyke nets (n = 6), 2-m fyke nets (n = 6), and 12 m x 6 m surface trawls (n = 20). We deployed the gear using standard methods (Murphy and Willis 1996). Mesh size for the 4 net types ranged from 1.59 mm (beach seine bag) to 19 mm (2-m fyke nets). We identified American Shad by mandible morphology (Munroe 2000, Weiss-Glanz et al. 1986) and measured total length (mm) of each fish caught. We collected salinity data using 2 independent methods. First, we collected salinity data throughout the study site during a mobile survey in the Penobscot estuary on 25 July 2012. The survey was conducted in an upstream zig-zag pattern, moving north in the direction of the tide, approximately from 4.5 rkm south of Verona Island to Bangor, ME (rkm 38.85). We used a data-logging multimeter (YSI model 6920; Yellow Springs Instruments [YSI], Yellow Springs, OH) deployed on a rigid frame attached to the boat at a depth of 0.5 m. We collected data once per minute on the calibrated multimeter for the entire survey. We downloaded data to a computer using EcoWatch Lite software (YSI 2015). We then imported the data into ArcGIS Version 10.2 and performed an inverse-distance–weighted interpolation using the Spatial Analyst Extension toolbox to estimate salinity between points for the entire study area (ESRI 2015). Second, we retrieved salinity data from the Northeastern Regional Association of Coastal Ocean Observing Systems website (http://www.; accessed on 11 March 2015) from Penobscot Bay Buoy F01 (44°3'21"N, 68°59'48"W; measurements taken at a depth of 1 m and recorded every 30 minutes) for the month of July to provide greater temporal coverage to the salinity data we collected. Results We captured 100 juvenile American Shad in the Penobscot estuary between 10 July and 23 August 2012 using all gear types (Table 1). The length distribution was bimodal, and the modes did not overlap (Fig. 2). The size range of fish was 18–112 mm (n = 73) in the lower mode and 154–203 mm (n = 27) in the upper mode. Twenty-three of the American Shad in the lower mode were 25 mm or smaller and therefore are classified as premetamorphic. All of these premetamorphic American Shad were captured using beach seines at 4 sites between rkm 28 and rkm 36 on 24 and 25 July (Fig. 3). Twenty-seven were 150 mm or larger, and were presumably age-1 American Shad. Northeastern Naturalist Vol. 23, No. 3 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 371 Salinity levels recorded during the hydroacoustic survey on 25 July ranged from 0 to 30.2 ppt. On 25 July, all locations upstream of rkm 13.50 had salinity levels less than 20 ppt (Fig. 3). On 25 July, all locations upstream of 3 km south of Verona Island had salinity levels less than 30 ppt (Fig. 3). Penobscot Bay Buoy F01 recorded salinity levels between 18.5 and 31.3 ppt during July 2012. On the 2 days that premetamorphic American Shad were captured, salinity at the Penobscot Bay buoy was consistently abov e 30 ppt. Discussion Salinity levels of 20 ppt or greater are detrimental to premetamorphic American Shad, resulting in significant mortality after several weeks of exposure, while salinity levels of 30 ppt or greater are lethal to premetamorphic American Shad (Crecco et al. 1983, Jia et al. 2009, Liem 1924). All of the premetamorphic American Shad captured in this study were caught at least 10 km upstream of these areas of high (>20 ppt) salinity. Therefore, our results demonstrate that these premetamorphic American Shad sampled in the lower Penobscot River in 2012 likely originated from successful upstream spawning events. This finding may be somewhat surprising given that Veazie Dam was essentially a complete barrier to upstream migration for adult American Shad (with only 16 American Shad passing Veazie Dam from 1978 to 2013; R. Dill, MDMR, Bangor, ME, pers. comm.). These data suggest that fish made effective use of the relatively limited availability of spawning habitat in the Penobscot River (roughly 15 km) prior to the removal of Veazie Dam in 2013. Our results also demonstrate prolonged and persistent use of the lower Penobscot River and estuary by juvenile American Shad. Our observations support Table 1. Number and stage/size of American Shad captured in July and August 2012 by gear and approximate location of capture. Month n Stage/size Rkm Gear July 4 Premetamorphic 27.66 Seine 5 Premetamorphic 31.00 Seine 14 Premetamorphic 31.57 Seine 1 26–112 mm 18.86 Seine 8 26–112 mm 27.66 Seine 21 26–112 mm 31.00 Seine 6 26–112 mm 31.57 Seine 1 26–112 mm 36.34 Seine 1 >150 mm 1.00 Trawl 1 >150 mm 11.24 Trawl 2 >150 mm 14.60 Trawl 22 >150 mm 17.00 Trawl August 1 26–112 mm 15.98 Seine 3 26–112 mm 21.75 Seine 1 26–112 mm 25.9 1-m Fyke 5 26–112 mm 27.66 Seine 1 26–112 mm 14.60 Trawl 1 >150 mm 14.60 Trawl 2 26–112 mm 17.00 Trawl Northeastern Naturalist 372 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 Vol. 23, No. 3 Figure 2. Length–frequency of juvenile American Shad captured in July and August 2012. Solid vertical line at 25 mm indicates length at which all smaller American Shad are premetamorphic. Northeastern Naturalist Vol. 23, No. 3 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 373 Figure 3. Catch of premetamorphic American Shad, 24 and 25 July 2012, with salinity data from transect survey on 25 July 2012. Circles indicate beach seine sites, and “n” indicates number of premetamorphic American Shad captured at each site. The dotted line indicates the first occurrence of salinity of 20 ppt encountered while moving downstream, and the horizontal solid line indicates the first occurrence of salinity of 30 ppt encountered while moving downstream. Northeastern Naturalist 374 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 Vol. 23, No. 3 several inferences by Limburg (1998), who used biochemical tracers to infer habitat use and life-history variation of river herring and American Shad in the Hudson River. In particular, Limburg (1998) noted that some yearling American Shad, Alosa aestivalis (Mitchill) (Blueback Herring), and Alosa pseudoharengus (A. Wilson) (Alewife) migrate in the springtime from marine or strongly estuarine waters up the Hudson River, concurrently with the runs of adult alosines. Additionally, she found that some yearling Blueback Herring and Alewife appear never to have left the Hudson drainage basin for marine waters and that yearlings of all 3 species appear to remain in the upper tidal freshwater Hudson River well into the summer. Limburg (1998) referred to both of these behaviors as “anomalous” migrations. Recent evaluations for 2 closely related species, Alewife and Blueback Herring, however, reveal that repeated movements from marine to freshwater habitat (even for age-0 fish) may not be uncommon in undammed systems (Gahagan et al. 2012). In fact, Limburg and Turner (2016) found that these movements are common for Blueback Herring in the Hudson River. It remains unclear whether American Shad that use the Penobscot estuary migrate between the marine environment and back (i.e., crossing salinity boundaries), or if American Shad in the Penobscot River exhibit prolonged estuarine residence, or both. Thus, our results provide further evidence of potential “anomalous” (sensu Limburg 1998) migration patterns for American Shad occurring in the Penobscot River. Further work with stable isotopes (or other biochemical tracers) is needed to clarify the pattern and prevalence of these alternative life-history strategies, as was recently described by Limburg and Turner (2016) for Blueback Herring in the Hudson River . Contemporary abundance levels of American Shad in the Penobscot also remain unknown. However, information from Grote et al. (2014a, b) suggests that numbers of adult American Shad are substantial and potentially greater than the previous estimate of around 1000. In fact, documented returns in 2015 exceeded 1500 at the Milford Dam (; accessed on 6 August 2015). However, more work is needed to develop a quantitative estimate of American Shad abundance in the Penobscot River. This information is critical in evaluating various management measures (natural recolonization, artificial propagation, etc.) in the future (Bailey and Zydlewski 2013). According to MDMR and MDIFW (2009) “restoration of the American Shad population can be accomplished by allowing adults to pass upstream and spawn naturally, trucking adults to specific river reaches and allowing them to spawn naturally, stocking hatchery-reared fry or fingerlings in the river, or some combination of these measures.” Each of these options has differential fiscal costs and population benefits. In addition, Hasselman and Limburg (2012) outlined a suite of genetic (potentially irreversible) costs of artificial propagation. Regardless of the option or suite of options chosen, knowing the starting population is important when evaluating the likely recovery time of American Shad in the Penobscot River (Bailey and Zydlewski 2013). While recent estimates of adult abundance in the Penobscot River are imprecise, our results demonstrate that the building blocks for American Shad recovery are present. We believe this information is useful to managers weighing future management decisions. Northeastern Naturalist Vol. 23, No. 3 C.A. Lipsky, R. Saunders, and J.R. Stevens 2016 375 Our findings of successful spawning are also encouraging in terms of the prospects of successful recovery of American Shad in the Penobscot basin regardless of which management measures are pursued in the future. If American Shad can successfully pass Milford Dam (the lowermost dam remaining in the Penobscot River), West Enfield Dam, and Howland Dam’s newly created bypass, then they will have access to 730 river kilometers, which is 93% of their historic habitat in the Penobscot River (Trinko Lake et al. 2012). Recolonization of historic habitat by American Shad can indeed proceed quite quickly, particularly when a local stock is present (Pess et al. 2014). However, Haro and Castro-Santos (2012) and Pess et al. (2014) caution that consideration of downstream passage must be given sufficient attention, particularly for American Shad in the northern portions of their range. In short, the higher degrees of iteroparity at northern latitudes (Leggett and Carscadden 1978) coupled with serial spawning behavior suggest that poor downstream survival of adults (Leggett et al. 2004) as well as juveniles (Harris and Hightower 2012) may negate improvements to upstream passage. Thus, considerable uncertainty remains for recovery potential in the Penobscot River because there are no quantitative targets for downstream passage of alosines (either adults or juveniles) at any of the remaining dams in the Penobscot River. Acknowledgements We are grateful to M. Colligan, R. Dill, C. Enterline, T. Sheehan, and J. Zydlewski for formative discussions on this subject. We thank A. Borsky, K. Gallant, G. Labonte, P. Ruksznis, D. Sagawe, and M. Simpson for assistance with data collection in the field, and J. Kocik, M. Simpkins, and K. Curti for helpful reviews of previous versions of this manuscript. Literature Cited Bailey, M.M., and J.D. Zydlewski. 2013. To stock or not to stock? Assessing restoration potential of a remnant American Shad spawning run with hatchery supplementation. North American Journal of Fisheries Management 33:459–467. Crecco, V., T. Savoy, and L. Gunn. 1983. 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