Wednesday, July 22, 2009

Part 6: What will happen once the power plant shuts down?

The big question of many who have read this story: What will happen once the power plant shuts down?

At this point, a simple answer is best: the turtles will most likely be just fine - and scientists will continue monitoring them to make sure that will be the case.

Sea turtles have existed on Earth for over 100 million years, and the warm water ceasing to be discharged from the SBPP will change the habitat of that part of the South San Diego Bay; but there will still be suitable habitats in the bay for the turtles.

What we might see is a change in where we see the turtles, and when we see the turtles.

If the turtles change location, this would be a spatial shift; if the turtles change the time of year we see them, that would be a seasonal shift.

A likely spatial shift:
For the past 30 years, the sea turtles of San Diego have been highly concentrated in the discharge channel (see red section to the left) (Dutton & McDonald, 1992). But once the power plant shuts down, the draw to that particular area of the bay will be eliminated, and the turtles may be just as likely to be in other shallow-water areas of the south bay - not just the discharge channel.

A possible seasonal shift:
When the power plant is operating, the water temperature during the winter stays warmer than it would naturally. By eliminating the warm-water source, the temperature of the bay during the winter may be too cool. In response, the turtles may avoid the San Diego Bay during the winter, stay in shallow areas where the water is warmer, or we may even witness the turtles exhibiting "hibernation" as has been observed in lagoons of Baja (Felger et al., 1976; Hochscheid et al., 2007).

Likewise, when the power plant is operating, the water during the summer becomes too warm. We may begin to observe the turtles in the bay more during summer months, and less during winter months.

(See image of scenarios of seasonal shifts. Top row: Winter and Summer WITH operating power plant. Bottom row: Winter and Summer WITHOUT operating power plant. Icons represent possible shifts in turtle distribution.)

Continued research on this population of sea turtles will help answer these questions and ensure that this endangered species is protected for generations of San Diegans to enjoy.

Cited works:

Dutton, P. & D. McDonald. 1992. Ultrasonic Tracking of Sea Turtles In San Diego Bay. In J.I. Richardson and T.H. Richardson (Compilers), Proceedings of the Twelfth Annual Workshop on Sea Turtle Biology and Conservation (pp. 218-221). Washington DC: NOAA Technical Memorandum NMFS-SEFC-361.

Felger, R., K. Cliffton, & P. Regal. 1976. Winter Dormancy in Sea Turtles: Independent Discovery and Exploitation in the Gulf of California by Two Local Cultures. Science , 191 (4224), 283-285.

Hochscheid, S., F. Bentivegna, M. Bradai & G. Hays. 2007. Overwintering behaviour in sea turtles: dormancy is optional. Marine Ecology Progress Series, (340) 287-298.

Saturday, July 11, 2009

Part 5: The Likely Explanation

Picking up from Part 4, if the same number of turtles are still in the San Diego Bay, and they are not actively avoiding our nets, then 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. Here I explore this likely explanation for the turtle's apparent decline in catchability.

Green sea turtles are tropical animals, preferring warm water in the 70s (F). During the winter, the Pacific ocean along San Diego's coast can get quite cool, dropping into the 50s. During this time, the water in the San Diego Bay also drops, typically in the 50s and 60s - making the bay's water a bit cooler than the tropical sea turtles might prefer.

However, the South Bay Power Plant (SBPP), when it is running, discharges warmed water into the south end of the SD Bay, which creates a pocket of warmer water - sometimes up to 20 degrees (F), but usually closer to 8-10 degrees (F) warmer than the rest of the surrounding water. This discharge water (see figure to the left for representation) creates a warm-water refuge for the sea turtles, especially during the winter months (December-March) when the bay's water would otherwise be quite cold.

(The green line represents a general typical daily patters of previously tagged sea turtles - Seminoff, Lyon & Eguchi, 2006. The green circles represent where the nets are typically set and where turtles are known to feed.)

If the power plant is operating at levels lower than historical operating levels, then less warm water is being discharged into the bay. Operating since 1960 and scheduled to be shut down in the next few years, the SBPP uses dated technology and is gradually being phased-out. The impact of this policy decision on the population of sea turtles is that the warm water refuge (the pocket of warm water created by the plant’s discharge) is becoming smaller and its presence is becoming less predictable.

This means that the turtles have less incentive to concentrate in the discharge channel where NOAA scientists have always observed the turtles. The turtles have likely found other areas around the south end of the San Diego Bay that have shallow and therefore warmer water that also suit their needs. This is the likely explanation for why the catchability of these green sea turtles has been declining in recent years.

Stay tuned: Part 6 will go into more detail about the impacts of the SBPP shutting down and what it means for the local San Diego Sea Turtles!

Photo: NMFS Permit #1591
NOAA scientists attaching tags to green sea turtles captured in the discharge channel of the SBPP.