By Research Assistant Lucy Williams
The following is a brief update regarding the tilt-current meter (TCM) deployment on the Hurricane Island aquaculture site this past summer and fall.
Find the first post here: https://www.hurricaneisland.net/island-updates/2021/9/15/tilt-current-meter-tcm
Our goal with the TCM was to collect flow speed and direction data during a spawning event on our experimental aquaculture farm. Scallops broadcast spawn, meaning they send gametes (eggs and sperm) into the water for fertilization. There are many studies on what induces large-scale spawning events for scallops and these results can vary from location to location and year to year. Hurricane Island has been collecting data from our farm for three years regarding approximately when spawning occurs and possible environmental factors that induce spawning such as temperature and moon phase. Specifically, downwellings coinciding with temperature increases have been shown to correspond to spawning events for scallops (J. C. Bonardelli et a, 1996), but this hypothesis had not been tested on our site. So, we deployed our TCM on the customized anchor at the end of August until the end of October, a collection time chosen due to historical data about spawning events on our farm.
How do we know when scallops are spawning? We collected scallops from our aquaculture site to dissect and weigh them in order to calculate what is called gonadosomatic index (GSI), the ratio of wet gonadal mass to the total wet body mass without the shell. The GSI essentially tells us if the scallop's in our sample aquaculture population are "ripening" to spawn or if they have spawned already. What we are looking for is a steady increase followed by a sharp drop in average GSIs from these samples, indicating that a spawning event has occurred.
Figure 2: TCM Data collected on Hurricane Island Aquaculture Site. Gray lines refer to exact data collected using a TCM device. The blue solid line is a viewing aid to show trends in data, calculated with a generalized additive model. Red dotted line refers to the maximum speed calculated during sampling, 19.76 cm/s, which occurred at 2021-09-10 15:55:00 EDT.
Figure 1b shows us a sharp drop in GSI in the beginning and middle of September, the weeks of September 1st through September 20th We assume that a spawning event occurred within that time frame. Figure 2 is the TCM data from the spawning weeks observed in the GSI data.
Figure 3: Daily average temperature at depth July - October at Hurricane Island Site. Sources for temperature data collection include Sonde, HOBO, and TCM devices. Shape type ( ●, ▲, ◼) indicate which device provided averages on a given day with multiple samples taken on some days.
We see that we have a local maxima during the second week of September. In fact, this is the maximum speed collected during the entire deployment from August to October. This maximum flow speed occurs during the middle of what we expect to be a spawning event. Whether this peak flow speed influenced scallop spawning is still yet to be determined. In addition, temperature is still missing from this analysis at this time. Our next steps are to look at temperature data during this time period and compare to surrounding months as well as recreate this experiment next summer and compare results.
Early stage temperature analysis suggests that daily temperature at the site peaked at a similar time as the spawning event. See in figure 3. However, peak occurs at the beginning of September, but daily average temperature begins to cool off into the weeks we believe fall under our spawning timeline. Potentially, this temperature turn from warming to cooling, coinciding with a large temperature drop from one sample to another suggests that the temperature change is more indicative than maximum temperature. Further experimentation with temperature at depth will be needed this coming summer to identify possible temperature and flow downwellings.
Works Cited
Bonardelli, J. C., et al. “Relation of Spawning of the Giant Scallop, Placopecten Magellanicus, to
Temperature Fluctuations during Downwelling Events.” Marine Biology, vol. 124, no. 4, 1
Feb. 1996, pp. 637–649., https://doi.org/10.1007/bf00351045.
The Tilt Current Meter is produced by Lowell Instruments. More info can be found on their website
