Building upon the 2007-2009 DEI study, this research project examined wild sea scallop enhancement methods for the fishery in eastern Maine through the culture and wild collection of scallop juveniles, intermediate grow-out methods, predator prevention and control, and environmental protection and control in discrete bottom areas in eastern Maine.
In 2009, when the proposal was submitted to NOAA-NMFS, sea scallop landings in coastal Maine (665,758 lbs.) were nearly 95% lower compared to a twenty-one year average from 1980 to 2000 (11,824,110 lbs; Fig. 1). Since 2009, statewide and site-specific closures to the coastal fishery through emergency rule-making by the Maine Department of Marine Resources have coincided with an upswing in landings. For example, landings in 2013 were the highest (3.5 million pounds) since 2001. The long-term effect of closures, however, is poorly understood because the mechanism responsible for the decline in landings is unknown. That is, if overfishing is the key to the decline, then closures that are geographically strategic should pay dividends. However, if the mechanism is related to an increase in predation or in recruitment failure, then closures would likely not be effective. Further, short-term (2-4 years) closures that coincide with concomitant increases in landings may simply represent a subsidy in the growth in biomass of sub-legal animals that remained on bottom when the fishery was closed.
Towards the goal of developing strategies to enhance the sea scallop fishery to increase production of sea scallops locally.
The study began in Western Bay off Great Wass Island in Beals in August 2010 by deploying 500 spat collector bags among ten shallow-water (< 20 m) and ten deep-water (> 25 m) sites. Similar attempts to collect wild spat occurred on three other occasions in the same vicinity from 2007-2009. In late April, 2011, the bags were collected; however, only 245 were found (49%). The amount of spat per bag was very low ca. individuals/bag. This density is two orders of magnitude less than that observed by Canadian researchers in waters of Passamaquoddy Bay, Nova Scotia, and the Northumberland Strait. The results were consistent with previous attempts to collect sea scallop juveniles in eastern Maine, and these methods cannot be recommended as a viable approach to obtain sufficient quantities of spat for stock enhancement or aquaculture trials.
In June 2012 the scope of the study was modified down to the objective of developing culture methods to reliably produce millions of sea scallop juveniles for stock enhancement purposes. Over a 30- month period, approximately 15 different spawnings and laboratory trials were conducted. During that time, we focused on: 1) varying the size of the culture tanks; 2) microalgal diets for both broodstock and larvae; 3) seawater temperature (ranging from 8oC to 14oC); 4) UV-treated seawater vs. physically filtered seawater through a 1-micron bag filter vs. using sodium hypo- chlorite (bleach) and then neutralizing with sodium thiosulfate; 5) handling (using a 2-day vs. a 4-day drain-down schedule); 6) initial scooping of trochophores from the water column 2 days after spawning rather than draining larvae onto 44μm mesh sieves; and, 7) bacteria (using a broad-spectrum antibiotic vs. no bacterial treatment at all). Unfortunately, no method consistently produced the millions of sea scallop spat necessary for stock enhancement work. Typically, a pattern of successful broodstock conditioning and spawning led to the production of tens of millions of fertilized larvae that would survive well for the first two weeks of culture, then slowly (and sometimes rapidly) decline to a few thousand in the culture tanks prior to metamorphosis.
In future efforts, the direction of work should be to examine the effects of different algal diets on growth to metamorphosis under culture conditions.
