Thursday, June 18, 2009

Part 4: Why is "catchability" declining? What is changing?

Previously we noted that it seems as though the green sea turtles in the San Diego Bay are getting harder to catch, see the number of turtles expected to be caught for each of the past 7 years in the chart to the left.

Three possible explanations for this were presented:

1) The population of sea turtles in the SD Bay is getting smaller
2) The turtles are changing their behavior and are adjusting & avoiding the nets
3) The distribution of the sea turtles in the SD Bay is changing

So, now to take a closer look at these possible reasons for the decreasing catchability. First, we found that the population of resident sea turtles in the San Diego Bay is not getting smaller, it is, in fact, remaining stable. There are approximately 60 green turtles - including new "fresh from the open ocean" juveniles - that feed on the eelgrass in the SD Bay (J. Seminoff & T. Eguchi, pers. comm., 2009). Most of these turtles are thought to be part of the breeding population from Mexico, and they are endangered. Given that the population is stable and NOT decreasing, this is not the reason for the turtles becoming harder to catch.

Second, it is very unlikely that the turtles are actively avoiding the nets used by NOAA scientists to catch the turtles during these monitoring days in the field. The monofiliment that these live-entanglement nets are made from is nearly invisible underwater, plus, the water in the discharge channel (where all of the nets are set) is very murky and has poor visibility. Therefore, this is also a very unlikely reason for the declining catchability.

Finally, it is possible that the turtles have changed their distribution in the south part of the SD Bay - that is, they may have changed where they hang out (rest, sleep, eat) - which could explain why fewer turtles are being caught by NOAA scientists when the nets stay in the same place but the turtles are elsewhere in the bay.

The next post, Part 5, will explore this likely explanation for the turtle's apparent decline in catchability.