Shrimp population studies; West Hackberry and Big Hill brine disposal sites off southwest Louisiana and upper Texas coasts, 1980-1982
The Strategic Petroleum Reserve Program involves the creation of crude oil storage capacity in salt caverns. As a by-product of solution mining techniques used to create this storage capacity, large quantities of brine are discharged into the Gulf of Mexico. Because of the size of the proposed discharges, there is concern that such disposal may adversely impact the shrimp fishery in the region. This report deals with the assessment of potential impacts of the discharge of brine at West Hackberry and Big Hill disposal sites on the brown and white shrimp fisheries in the southwest Louisiana and upper Texas coastal region. Shrimp stocks are known to fluctuate considerably in abundance as a result of climatic variability. Because of this, it is important to distinguish between changes in shrimp abundance that are climatically induced and changes that may be due to brine disposal. Although it is desirable to understand in detail how climatic variability can cause changes in certain of population parameters, such as growth and mortality, fecundity, post larval recruitment, age at maturity, etc., in practice not enough information is available on either the shrimp populations or climatic conditions to develop mechanistic models which could be used for predictive purposes. This study attempts to identify plausible empirical relationships which may be used to build predictive models. The methods developed herein utilized information on historical trends in shrimp landings, and quarterly climatic factors for predicting fluctuations in landings of white and brown shrimp. The three basic analytical techniques used are: regression equations using principle components, analysis of covariance models, and Autoregressive Integrated Moving Average (ARIMA) models. These three techniques provide realistic bounds on the amount of variability that can be expected in shrimp landings once environmentally induced fluctuations and historical trends in landings have been taken into account. The following generalizations are possible on the basis of the shrimp life histories, and the results of using the regression equations for predicting annual landings. For brown shrimp, the major concentrations occur off Texas. It is hypothesized that relatively strong eastward Ekman transport during the winter (Jan. - Mar.) favors the movement of larval brown shrimp toward nursery grounds on the eastern end of their range. However, once on nursery grounds, juvenile brown shrimp may not survive if springtime river discharges are high. In agreement with these hypotheses, the combination of strong eastward Ekman transport in the winter, and low springtime river discharges correlated with the largest catches for brown shrimp in statistical area 18. In addition high spring river discharge seems to reduce catches in this area, regardless of whether Ekman transport was favorable or not. Landings from the previous year were found to be a major predictor for white shrimp landings in statistical areas 17 and 18. This may be due to a general increasing trend in white shrimp landings in these areas over the period 1960-1977. Increases in white shrimp landings were found to be positively correlated with high springtime river discharges. Strong Ekman transport to the east in the spring was correlated with decreased landings for white shrimp in area 17, while strong northeast Ekman transport in the summer had a similar effect in area 18. In the spring and summer white shrimp spawn offshore, move into nursery grounds, and then start their offshore migration as adults. Since heaviest concentrations of white shrimp occur in the soft mud and sand bottoms off the coast of Louisiana, strong transport toward the east during the spring and summer tends to favor transport into statistical areas 17 and 18 from western areas with relatively lower white shrimp density. This could explain the observed decreases in landings mentioned above.