Watercraft related Manatee Mortality

Based on my analysis of the available empirical data I have repeatedly stated that boat slow speed zones in murky waters are ineffective and do not provide increased manatee protection.

This is based on the fact that historically in Florida the average watercraft-related manatee mortality percentage of total manatee mortality has remained relatively constant at ~ 25% over the past 3 decades or more . If the zones were effective, the average percentage of watercraft-related mortality should have reduced after the zones were implemented, but it hasn't.

Some biologists have argued that the increasing number of registered vessels has offset the benefits from the slow speed zones, but that seems to me to be rather self serving since no empirical data that supports such a hypothesis has been provided. The FWC and FWS continue to ignore the fact that the average watercraft-related mortality trend line is FLAT relying on some mysterious or miraculous force to consistently and exactly offset the speed zones' benefits. Unfortunately, the history of vessel registration does not support such a thesis.

If you want to isolate the effect of one variable to another then normalizing data is the preferred approach. A simple percentage is a normalization process. Some other examples of normalized variables are apples grown per acre, human deaths per thousand aircraft flight hours, and car accidents per thousand miles traveled.

When annual watercraft-related manatee mortalities are normalized (divided) by annual vessel registrations some further insight regarding the assumed correlation is garnered. The attached table shows at the State and 2 counties levels the number of watercraft-related manatee mortalities for a period from 1984 to 1991 which is before most of the slow speed zones were implemented. Similar data is also available from 2000 to 2007 which covers the period after most of the slow speed zones were implemented which now amounts to ~ 23% of all manatee inhabited State waters. In Brevard and Lee Counties it's ~ 33%!

The average number of watercraft-related manatee mortalities per 10,000 registered vessels for the period before most slow speed zones existed is significantly LOWER than the same computed average after the zones had been implemented, i.e., 0 .63 Vs 0.87 which amounts to ~ 38% INCREASE. That would seem to indicate that something other than increasing vessel numbers are affecting the watercraft-related manatee count each year. Could it possibly be the manatee population? An increasing watercraft-related manatee mortality trend or for that matter the total mortality from most causes is indicative of an increasing manatee population.

Some have said that the zones aren't being enforced and maybe that's the reasons why the slow speed zones are ineffective. However, after some slow speed zones were implemented there was an aggressive on water law enforcement activity in the Spring and Summer of 2002, 2003, and 2004. A FWS study stated, "... they concluded that the lower number of violations in 2004 was due to improved boater compliance." Interestingly, the on-water law enforcement and the noted boater compliance coincided with HIGHER ratios of watercraft-related manatee mortalities per 10,000 registered vessels than before the zones existed.

Conclusion: The State vessel registration data when used to normalize the manatee mortality data does not support the claim that increased number of vessels are offsetting the benefits from slow speed zones. All the available data supports the position that the most probable contributor is an increasing manatee population. In murky waters (most of Florida) slow speed zones do not provide added protection for the manatee and may in fact exacerbate the problem of vessel-manatee collisions.

One wonders why the FWC and FWS do not conduct or publish such analysis. I suspect the FWS and FWC will continue to ignore the facts, but someday the right law suit will convince a court that the wildlife agencies actions in this regard have been arbitrary and capricious.

Captain Tom McGill, P.E. 7/24/08