Estimation of fisheries impacts due to underwater explosives used to sever and salvage oil and gas platforms in the U. S. Gulf of Mexico

Abstract

From the Executive Summary

According to data from the National Marine Fisheries
Service (NMFS) Platform Removal Observer Program which includes
removals in both federal and state waters, from 1989-98 a total
of 958 structures were salvaged using explosives for an annual
average of roughly 96 structures. One obvious consequence of
using explosives is a negative impact on fish. There has
previously been no attempt to quantify the impacts of explosive
platform removal on fish populations. Of special concern is the
commercially and recreationally important red snapper (Lutjanus
campechanus) which occurs at many of these structures. The red
snapper continues to be the subject of intense government
regulation as this species is severely overfished and there are
significant problems in the long-term viability of the stock.

Fishery managers attempt to track the size and status of
stocks using mathematical equations which include variables
relating to recruitment and mortality . The results of such stock
assessment analyses provide managers with critical information
needed to manage fisheries . This study quantifies the mortality
of fish species resulting from explosive platform removals . For
the first time, mortality estimates from platform removals were
used in stock assessment analyses to determine the relative
importance of this mortality compared with other sources of
mortality such as commercial and recreational fishing, trawl
bycatch, and discards. As a result, stock assessments may be
improved through addition of this new parameter into stock
assessment equations.

The most severely impacted fish species at explosive
structure removals in order of abundance were Atlantic spadefish
(Chaetodipterus faber), blue runner (Caranx crysos), red snapper
(Lutjanus campechanus), and sheepshead (Archosargus
probatocephalus). These four species accounted for 86% of
estimated mortality. Numbers of all other impacted species were
far below those of the top four. Of the species encountered in
these field studies, only red snapper, gag and red drum have
stock assessments conducted on them by the National Marine
Fisheries Service. For red snapper, even when the mortality
estimate was doubled, impacts were estimated to be small, well
within the variation of our current assessments, and would not
alter current determinations of status or current management
recovery strategies. Similarly, current methods of assessment
would not detect the even smaller changes in magnitude of gag and
red drum. Results indicated no significant difference in
estimated mortality of red snapper by depth, longitude, platform age, season, surface salinity, and surface temperature in the
study area (14–32 m)during May to September. These analyses
suggest no appropriate strata for expansion of mortality data to
the greater Gulf of Mexico and indicate that platforms in the
water depths studied can be included in a single group for the
purpose of estimating fish mortality due to explosive platform
removals. Although the effects of structure complexity on fish
abundance was not an objective of this study, unpublished data
from the National Marine Fisheries Service indicated structure
complexity may directly influence observed mortality. This
parameter was integrated into the sensitivity analysis for stock
assessment. A significant difference in red snapper length at
removals in 20–30 m water depths vs those at shallower and deeper
depths was also incorporated into the analysis.
Future impacts to the red snapper stock were predicted based
on forecasts of future structure removals reported by Pulsipher
et al . (in press). Estimates of future mortality were higher than
current estimates but less than the doubled value of current red
snapper mortality which was used in these stock assessment
analyses. Consequently, future red snapper mortality estimates at
explosive structure removals fall within the variation of our
current assessments. Given the assumptions used in these
forecasts, predicted future mortality would not alter current
determinations of stock status or current management recovery
strategies for red snapper. However, should future facts alter
the validity of these assumptions, then these predictions should
be revised accordingly.

Three important caveats should be remembered when
interpreting these results. First, species composition and
abundance can change in water depths deeper than those
encountered during this study. Second, sample size was small,
only nine platforms out of more than 4,000 structures present in
the U .S . Gulf of Mexico. Finally, all sampling was conducted
during the months of May through September .