Quantifying the Relative Abundance of Juvenile Atlantic Sturgeon, Acipenser oxyrhychus, in the Hudson River
Studies have shown a significant decline in populations of Atlantic sturgeon (Acipenser oxyrhynchus) and shortnose sturgeon (Acipenser brevirostrum) across the Eastern seaboard, particularly in the Hudson River. This study is the first to monitor the Atlantic sturgeon population in the Hudson River since the closure of the commercial sturgeon fishery in 1996. Relative abundance sampling for the sturgeon was done in Haverstraw Bay in the Hudson River (Kilometers 58 to 66), using anchored gill nets. During the two-year span in which this study took place, the sturgeon catch per unit effort, or CPUE, improved for both the Atlantic sturgeon (to 0.295 in 2009, from 0.082 in 2008) and the shortnose sturgeon (to 0.085 in 2009, from 0.056 in 2008), suggesting potential population increases for both species. It was determined that the greatest impact on the CPUE was distance from the salt front. Juvenile Atlantic sturgeon were caught more frequently when we sampled below the salt front, in conditions of higher salinity. In contrast, shortnose sturgeon preferred water with lower salinity and were found more frequently when sampling occurred above the salt front. These first two years of data are important indices for future years and will help to determine changes in surgeon populations and the effect of salinity on sturgeon populations.
The U.S Government currently lists 1,311 animals as endangered/threatened species, 139 of which are fish, more than any other taxonomic group (USFWS 2008). With extinction rates so extraordinarily high, it is vital to understand the connection between fish and their environments. Rivers and estuaries are vital parts of the ecosystem that many fish species use for spawning and nursery habitats, allowing juveniles to grow and develop in a protected area. The Hudson River estuary is an essential element in the life cycle of the Atlantic sturgeon (Acipenser oxyrhynchus) (Figure 1) and the shortnose sturgeon (Acipenser brevirostrum) (Figure 2). Understanding how these fish populations fluctuate or change will help agencies determine the best management strategies to prevent further population declines.
In 1967, based on its declining population numbers, the shortnose sturgeon was listed for the Endangered Species Preservation Act, which preceded the Endangered Species Act of 1973. Its decline was caused by habitat loss after dam construction, damage to its habitat from dredging and pollution, and overharvesting of adults (Kynard 1996). In 1988, the Atlantic sturgeon was added to the National Oceanic and Atmospheric Administration's (NOAA) Species of Concern list because of declining populations, for similar reasons (Smith 1997), with the intent of protecting the remaining populations from further harm and allowing them to naturally rebuild over time. Critical sturgeon habitats were designated, and recovery plans were implemented. The 2007 Atlantic Sturgeon Status Review Team (ASSRT) recommended that the New York Bight, Chesapeake, and Carolina sturgeon subpopulations be listed as threatened under the Endangered Species Act (ESA) as they have a high risk of becoming extinct within the next 20 years. Shortnose sturgeon populations are showing signs of strong recovery within the Hudson River, although most Southern populations show little increase in abundance (Secor 2005).
Both the Atlantic and shortnose sturgeon are anadromous, each with life spans of approximately 60 years. The Atlantic sturgeon is larger and can reach lengths of 4.5 meters and weigh up to 370 kilos, making it the largest fish to inhabit the Hudson River estuary (Bain 1994). Conversely, the shortnose sturgeon is the smallest of the North American coastal sturgeon (Secor 2005), and can attain a maximum length of 1.2 meters and a weight of 24 kilos. Both sturgeon species inhabit estuaries ranging from Labrador in the northernmost reaches of the Atlantic coast to the St. Johns River in Florida in the South (Figure 3) (Smith 1985).
It is known that juvenile Atlantic sturgeon will grow and remain in their natal river or estuary for two to six years before migrating to marine waters. The subadults (older than 6 years) can remain in marine waters for more than a decade before returning to rivers and estuaries to spawn. Age of maturity is affected by water temperature and the sex of the fish. In the Hudson River, the male Atlantic sturgeon begins to mature around age 11, while females begin to mature around age 20 (Smith 1985). Male Atlantic sturgeon may spawn each year, while females may spawn only once every three to five years (Sweka et al. 2006). Because of this complex life cycle, the early juvenile stage often provides the best opportunity for population studies because it is the only age class that remains in a single body of water (Bain 1999).
Shortnose sturgeon mature between the lengths of 45 to 55 centimeters throughout their range. However, the age at which they reach this length depends upon their latitudinal position, with northern fish growing at slower rates than southern. Shortnose sturgeon spend most of their lives in coastal river and estuaries, with only a few migrating into coastal waters.
Since the late 1800s, overfishing has plagued the Hudson River commercial fishery because of the demand for sturgeon meat and caviar (Smith 1985, Bain et al. 1999). The Atlantic sturgeon commercial fishery can be divided into time periods marked by high- and low-harvest cycles (Figure 4) (Sweka et al. 2006).
While landings of shortnose sturgeon were prohibited in 1967 with their listing on the Endangered Species Preservation Act, fishing of Atlantic sturgeon continued. In 1996, the New York State Department of Environmental Conservation placed a moratorium on the Atlantic sturgeon fishery in the Hudson River. The moratorium bans commercial and recreational fishing for a mandatory 40 years to ensure that 20 protected-year classes will enter the spawning population (Sweka et al. 2006). In 1998, the Atlantic States Marine Fisheries Commission followed suit by placing a coast-wide moratorium on the sturgeon fishery (ASMFC 1998). This moratorium is vital to the protection and rebuilding of Atlantic sturgeon stocks for future generations.
Two previous studies have produced population estimates for juvenile Atlantic sturgeon in the Hudson River. W.L. Dovel and T.J. Berggren (1983) estimated the 1976 population at 25,647, and later Peterson et al. (2000) estimated the 1994 population at 4,314, which was an alarming decline in population. Because these estimates were made while the commercial sturgeon fishery was still active, it remained unknown how the population has responded since the 1996 fishing moratorium.
Thus, the purpose of this study is to evaluate how the juvenile Atlantic sturgeon population has responded to the 1996 closure of the Hudson River fishery and the coast-wide moratorium. The specific objectives of this study were threefold:
(1.) To quantify the relative abundance of juvenile Atlantic sturgeon and shortnose sturgeon in Haverstraw Bay on the Hudson River.
(2.) To determine the effect of environmental factors on both the Atlantic and shortnose sturgeon catch per unit effort (CPUE) in Haverstraw Bay.
(3.) To report and document the physical characteristics of juvenile Atlantic and shortnose sturgeon stocks.
I hypothesized that the Atlantic sturgeon population in the Hudson River would be stabilizing; that the Atlantic and shortnose sturgeon catch per unit effort (CPUE) would be affected by environmental factors, including water salinity, distance from the salt front, water temperature, and dissolved oxygen levels; and that a shift in average total length and weight for both the Atlantic and shortnose sturgeon populations would be observed from Year 1 to Year 2.
Sampling for Atlantic and shortnose sturgeon took place in Haverstraw Bay in the Hudson River (Kilometers 58 to 66) during the months of March and April in both 2008 and 2009. Haverstraw Bay, the widest part of the Hudson River, is 8 kilometers long by 5.5 kilometers wide, and ranges from Croton Point (Km 58) to Stony Point (Km 66). The sturgeon were sampled in the soft/deep areas of Haverstraw Bay, as these areas were determined to produce the greatest CPUE for juvenile Atlantic sturgeon in a previous study (Sweka et al. 2006). A soft/deep habitat is a location that has a soft substrate and a water depth greater than six meters. The soft/deep habitat was identified by benthic mapping of Haverstraw Bay (Figure 5) (Lamont Doherty Earth Observatory), and then random GPS points were chosen within the area as sites for net placement.
Anchored gill nets measuring 61 meters by 2.4 meters, with stretch mesh sizes of 76, 102, and 127 millimeters, were employed for sampling. These mesh sizes were found by prior studies to effectively sample a large size range of juveniles (Bain et al. 1999, Sweka et al. 2006). The nets were placed in sets perpendicular to the shore and parallel to one another, approximately 30 meters apart. Nets were set during all tidal phases and only during daylight hours. Each net was fished for two hours before being retrieved and examined for sturgeon. The bycatch (white perch, striped bass, etc…) was recorded and then immediately released back into the river.
Sturgeon Measurements & Tagging
Captured Atlantic and shortnose sturgeon were immediately removed from the net and placed into a live well prior to data collection. The sturgeon were weighed to the nearest gram and measured for total length, fork length, eye width (Figure 6), and mouth width to the nearest millimeter (Figure 7).
All sturgeon were tagged in order to aid in tracking using a passive integrated transponder tag, or PIT tag, from Biomark, and an external orange Carlin tag. The PIT tag was injected under the skin, giving each fish a unique ID number that could be read if/when the fish is recaptured to track its individual movement and monitor its growth. The external Carlin tag was fixed to the rear dorsal fin and is designed to be visible to anyone who may catch the fish, instructing them to contact the U.S. Fish and Wildlife Service coastal sturgeon-tagging database (Figure 8). Immediately following the tagging, the sturgeon was released back into the river. Throughout sampling, no fish injury or mortality occurred.
To evaluate habitat preference, water chemistry data was collected using a YSI 556 multiprobe system (Figure 9). The water temperature, dissolved oxygen, specific conductance, total dissolved solids, pH, and salinity were measured at each net set (one to two feet off the bottom). Distance from the salt front was taken from the U.S. Geological Survey website, and is calculated using complex models that use current, tidal cycles, and precipitation to mathematically determine the location of the salt front.
Relative abundance, a measure of population, was calculated based on the catch per unit effort (CPUE) trend over multiple years. CPUE is a standardized formula used in the calculation of relative abundance (Sweka et al. 2006). During the course of the study, CPUE was calculated for each net set and as an average for the two seasons (2008 and 2009). CPUE is defined as the number of fish caught divided by effort and multiplied by the number of nets (Figure 10).
All the statistical analysis was done using Microsoft Excel 2007 to determine if relationships exist between the CPUE and water temperature, dissolved oxygen, specific conductance, total dissolved solids, pH, salinity, and distance to the salt front.
During the spring 2008 sampling season, a total of 68 juvenile Atlantic sturgeon and 48 shortnose sturgeon were captured. During the spring of 2009, 196 juvenile Atlantic sturgeon and 55 shortnose sturgeon were captured. The shortnose sturgeon captured were generally larger (both in total length and weight) than the juvenile Atlantic sturgeon. The average size of both juvenile Atlantic and shortnose sturgeon increased in 2009 from sampling in 2008 (Table 1).
Both the length and weight frequency graphs display similar shifts in size over the two years of sampling (Figure 11).
The CPUE for juvenile Atlantic sturgeon and shortnose sturgeon showed an increase between 2008 and 2009 for both species. Notably, the Atlantic sturgeon showed a considerable increase in mean CPUE, to 0.295 in 2009 from 0.082 in 2008 (P<.001), and the shortnose sturgeon rose to 0.085 in 2009, from 0.056 in 2008 (P=.083) (Table 2).
Distance to the salt front was correlated with the CPUE for both Atlantic and shortnose sturgeon. Atlantic sturgeon were caught more frequently in nets that were south of the salt front, in water with a higher salt content. Shortnose sturgeon displayed almost the opposite behavior; the CPUE was greater in nets set north of the salt front (Figure 12).
Water quality data was correlated with the CPUE of each net set, using a linear regression. Linear regression determines whether a chosen environmental value correlates significantly with an increase or decline in the CPUE. Inconsistent results were seen for most of the variables; water temperature, salinity, specific conductance, total dissolved solids (TDS), dissolved oxygen, pH, and dissolved oxygen did not display significant r or P values (all r< .11) when compared to CPUE (Table 3).
Compared to 2008, the 2009 sample size was larger in two ways: more individual fish were caught, and the average size of the fish increased. Both juvenile Atlantic and shortnose sturgeon caught in 2009 were larger by an average of 30 millimeters and 100 grams than those caught in 2008. In 2009, 196 juvenile Atlantic sturgeon were captured as compared to only 68 in 2008. The 2009 sampling season was significantly more productive as a result of fishing south of the salt front. In 2008, most nets were set north of the salt front, in freshwater where there was a smaller concentration of juvenile Atlantic sturgeon.
The environmental variables I measured were to determine if they had effects on the sturgeon populations, and to identify the ideal environmental conditions to maximize the CPUE. It was determined that distance from the salt front had the greatest impact on the CPUE of both juvenile Atlantic and shortnose sturgeon. Juvenile Atlantic sturgeon were caught more frequently when we sampled below the salt front, in conditions with greater salinity. Juveniles may prefer these brackish waters because their bodies need to acclimate to salt water before they move out to marine environments for the next decade of their lives. It is also possible that the area around the salt front provides a better source of food for juvenile Atlantic sturgeon. This remains to be investigated further; future research could employ lavage techniques to assess their diet in response to the salt front.
The shortnose sturgeon showed a nearly opposite preference regarding the salt front. Shortnose sturgeon were found more frequently north of the salt front, in the water with lower salinity. This is probably a result of their migration from marine waters to fresh waters to spawn near Albany. The shortnose sturgeon must become acclimated to the fresh water after spending the greater part of the year out in marine waters.
While our results show increases in CPUE for both Atlantic and shortnose sturgeon, caution must be exercised before drawing conclusions about population trends. The limitations of the current study include only two years of data, which prevents an accurate comparison of the CPUE over time, which is vital for calculating relative abundance. Environmental variables, specifically distance from the salt front, can skew the data, and it is important to consider this when looking at the average CPUE for the season. This study is also limited by the seasons; there is only a brief, one-and-a-half-month window in which sampling occurs. This greatly limits the number of possible net sets and makes it more difficult to catch large quantities of sturgeon.
Relative abundance sampling of the juvenile Atlantic sturgeon in the Hudson River displays a population trend over time. The current study is aimed at calculating relative abundance. In this study there is a 90% chance of seeing a 7% change in population numbers over 10 years. This is the second year of the study, and the calculated numbers will set a baseline for future years. The CPUE for both juvenile Atlantic and shortnose sturgeon will serve as a starting point for this study and other studies to come. In future years, a population increase may be seen as a result of the current moratorium on fishing.
Over a span of 30 years, ending in 1977, the General Electric Company (GE) discharged water containing polychlorinated biphenyls (PCBs) into the Hudson River at Fort Edward (Connolly 2000). Water quality throughout the Hudson River estuary has been found to be improving; concentrations of heavy metals have declined more than 50% over a period of 25 years. These reductions are a direct result of improvements in the control of waste-water discharge required by the 1972 Clean Water Act (Sañudo-Wilhelmy 1999). Currently, conditions are in place to support a healthy population of Atlantic sturgeon. The juveniles who were born during the fishing moratorium are now beginning to spawn, so I predict that further increases in sturgeon populations will be detected in future samplings.
In the future, a yearly sampling plan is recommended to monitor the Atlantic sturgeon population in the Hudson River through the end of the fishing moratorium, as well as after the moratorium ends to track fishing-related mortality. This yearly sampling plan is vital to maintaining a healthy sturgeon population; a population index each year will prevent further population loss. Along with annual sampling to obtain an accurate measure of relative abundance, recapture of individual fish will allow the calculation of a growth rate for the juvenile Atlantic sturgeon in the Hudson River. An accurate model of the growth rate of juvenile Atlantic sturgeon in the Hudson River is a new and valuable tool that could be used when future laws and regulations are drafted.
In the two-year span during which this study took place, Atlantic and shortnose sturgeon CPUE numbers improved, suggesting potential population increases. However, multiple years of data are needed to create an accurate population estimate of juvenile Atlantic sturgeon in the Hudson River. After two years of studying environmental variables in the river, the CPUE for both juvenile Atlantic sturgeon and shortnose sturgeon is most affected by the salinity of the water and distance from the salt front. Catch per unit effort for Atlantic sturgeon was highest just south of the salt front, in water with greater salinity. Shortnose sturgeon were more frequently caught in fresh water north of the salt front. Other environmental variables did not correlate consistently with the CPUE.
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