To determine the role that salinity plays on blue crab settlement, growth and survival we conducted a series of field surveys and experiments in the ACE Basin National Estuarine Research Reserve. This study was funded by Clemson University and the SC SeaGrant program and ran from March 2008 to March 2012. To monitor the response of blue crabs to changing salinity conditions, we sampled crabs at nine stations (1 being closest to St Helena Sound and 9 being closest to the northern ACE Basin boundary) in each of the three rivers (from left to right, Combahee, Ashepoo, and Edisto). These three rivers differed in the amount of freshwater discharge and thus, had different salinity profiles. We sampled these four rivers quarterly (March, June, September, December).
In order to measure water quality, we used a YSI multiprobe to measure temperature, salinity, dissolved oxygen and pH at each sampling station. We also counted crab pot buoys between each water quality station to estimate fishing effort. In order to measure crab abundance at each sample station, we placed four modified commercial crab pots (wrapped in mesh to retain juvenile crabs) at each station in the river and pulled them after a 4 hour soak. All crabs were returned to the field station for further processing. Every crab was sexed, weighed and measured to the nearest 0.1 mm across the width of the carapace (CW) from the tip of one lateral spine to the tip of the other lateral spine. Each crab was photographed and had a blood sample drawn and preserved for analysis of Hematodinium spp. infection. All crabs were then released unharmed back into the estuary.
These graphs show how water quality measures changed between seasons and years at the mid-point on the river (station 5). The x-axis are the 16 sampling dates beginning in June 2008 through March 2010. All four water quality measures differ by season with salinity and temperature showing peaks in June or September and dissolved oxygen showing peaks in December or March. Temperature and dissolved oxygen did not differ between the three rivers but salinity and pH did. The Combahee River had the highest salinity profiles and showed an increase in salinity from 2008 to 2012.
These graphs show the sum of blue crabs (all nine stations combined) by sex and size for all 16 sampling dates within each of the three rivers. Blue bars are males and red bars are females. Darker bars are juveniles and lighter bars are adults. The green line overlaying the graph is the salinity at the mid-point of the river (station 5). Clearly blue crabs are most abundant in the intermediate salinity Ashepoo River and least abundant in the low salinity Edisto River. There was no strong pattern of seasonal blue crab abundance with seasonal variation in salinity. Adult males were the only subclass of blue crabs whose abundance was correlated with salinity as more adult males were observed on low salinity sample dates.
In order to test the hypothesis that changes in salinity influence crab populations, we conducted one manipulative field experiment and three observational studies. To estimate the impact of salinity on predation of juvenile crabs, we tethered 20 juvenile crabs to a weighted line and placed them at four stations (1, 3, 5, 7) in each of the three rivers. To estimate the impact of salinity on disease, we used a PCR-based laboratory assay to identify which of the crabs surveyed were infected by Hematodinium spp. To estimate the impact of salinity on fishing effort, we compared the number of crab pots being fished to the salinity of each region of the three rivers. And finally to estimate the impact of salinity on blue crab larval settlement, we deployed and sampled larval collectors at four stations (1, 2, 3, 4) in each of the three rivers.
We found a significant negative relationship between salinity and relative predation. Juvenile blue crabs are eaten more frequently in low salinity stations and in the low salinity river. We found the opposite pattern with regards to disease. Crabs at high salinity stations and in the high salinity river had much higher levels of Hematodinium spp. infection. Neither fishing effort nor larval settlement showed any consistent patterns in relation to changes in salinity. Given that high salinity increases disease and low salinity increases predation, it is not surprising that blue crab abundance is highest at stations with intermediate salinity and in the river with intermediate salinity.
Given that crab abundance may be harmed by salinities that are either too low or too high, what is the consequence of droughts on blue crabs? During this four year study freshwater input to the ACE Basin hit historical lows in 2008, 2011 and 2012. As a result, the average salinities slowly increased throughout the study in all three rivers. Surprisingly, this caused a different response to crab numbers in each of the three rivers. In the low flow Combahee river, increasing salinity caused decreasing crab numbers likely due to increasing disease. In the high flow Edisto River, increasing salinity caused increasing crab numbers likely due to decreasing predation. But in the intermediate flow Ashepoo River, increasing salinity had little effect because salinities remained in the optimum range for blue crabs.
This figure summaries the findings of our four year study of the impact of salinity on blue crabs. Crabs due best at intermediate flow levels and intermediate salinity locations. When flow is decreased like during a drought, salinity increases and the impact can be negative through an increase in disease. This is likely to occur more in the lowest flow locations. However, during a drought the increase in salinity may also be positive by reducing predation. This is likely to occur only in the highest flow locations. The future of blue crab will depend on flows remaining in the optimal range to produce the largest salinity sweet spot across the greatest number of rivers.