Lessons from 2009
We learned a tremendous amount in the first year of the megalops collection study. We learned that volunteers make diligent researchers, can be a great source of local information, and have lots of good ideas for improving the study methodology. We learned the value of great volunteer coordinators. And we learned that 2009 was probably not a good year for megalops – especially in Hood Canal.
Despite the efforts of many volunteers, very few megalops were found in Hood Canal, Admiralty Inlet and the Strait of Juan de Fuca in 2009. The amount collected from north Puget Sound sites was higher but not as high as expected. The results are summarized on Table 1. The sample locations are depicted on the map. Those who have installed the free program, Google Earth, can view the results by opening the KML file.
Why we found so few megalops
The short answer is that we don’t know. The reasons could be due to methodology,the collection method or due to natural variation.
Possible method - related causes:
Timing: We got a late start and may have missed an early settlement in Hood Canal. However, we expected settlement to be greatest later in the summer but it was only in north Puget Sound. There, we collected the most megalops in late July and early August. In 2010, we plan to start collecting in May.
Sample locations: We stationed collectors where volunteers had access. Some of these may have been in locations where tidal or wind driven currents don’t tend to bring things close shore. We saw this in Bellingham Bay. Example: Taylor Dock had 26 megalops, while Coho Dock, only 1.4 miles away, had only 2 megalops collected.
The slope of the beach is another important factor. When waves hit sea walls or steep beaches, the energy is deflected back away from shore, pushing megalops away. Megalops would more likely be moved closer to shorelines which have gradually sloping beaches.
This year, we will be adding stations in locations where currents are more likely to concentrate megalops. This will be tricky, since a lot of the surface currents are wind driven, and the wind is not always very predictable.
Diversifying megalops collectors: We know from other studies and results in north Puget Sound that the Tuffy scrubber collectors work. But they are still just passive collectors. Megalops have to come across them and prefer to settle in them over other nearby habitat. This year, we will add light traps in some of the areas to draw them in. Megalops should go to these light traps literally like moths to a flame.
Possible natural causes of the low megalops collections
In 2008, a vast deposit of cast-off megalops shells washed up on some beaches in Hood Canal. In past years, reports of this type of event and other signs of megalops masses including swarms of megalops in the water and megalops fowling nets are fairly common. In 2009, only a small surface water swarm was reported in July in north Puget Sound. This, and the fact that low-to-no numbers of megalops were collected, strongly indicates that 2009 was just a year with low megalops supply. We are just beginning to learn how to find them, so discussions about the causes of low megalops numbers is purely speculation, however here are some ideas:
Maybe, high numbers of megalops washing in from the Pacific are a relatively rare occurrence. The prevailing surface current in the Strait of Juan de Fuca flows west. This may be why we have been spared, so far, an invasion of green crab. There are times when winds reverse the current, but these are relatively rare events occurring most likely in late summer.
Low dissolved oxygen in Hood Canal is also suspected to contribute to the decline in the crab population. But the low abundance of Dungeness megalops seems to have occurred Puget Sound wide to some extent. So low DO can’t be the only cause of low megalops numbers in Hood Canal. Notably, it was not a particularly bad year for low DO in Hood Canal.
We also have suspected that fishing impacts (mortalities on small males and females from repeated handling) plays a role in the low population and reproduction levels. But huge numbers of megalops were documented in Hood Canal in 2008 – just as catch appeared to bottom out. Similarly, fishing has been good in other areas where problems with handling mortality should have been a factor. Therefore, it is not likely that handling mortality is the primary cause of low recruitment.
Finding the cause of these fluctuations is one of the long range goals of this study, and we are just at the beginning. Our hope is that more locations, an earlier start and adding light traps to our arsenal of larvae collectors will help us get a better picture of the timing and distribution of megalops.