WINTER-TIME DISTRIBUTION AND ABUNDANCE OF COPEPOD 

 NAUPLII IN THE NORTHERN GULF OF MEXICO 



M J Dagg^, P B Ortner2, and F. Al-Yamani^ 



ABSTRACT 



Copepod nauplii were collected from continental shelf waters in 3 regions of the northern Gulf of 

 Mexico during winters between 1981 and 1984, off Cape San Bias, Florida, off the Mississippi River 

 delta, and off of Galveston, Texas. Some statistically significant iP < 0.05) patterns in the abundance 

 and distribution of nauplii were observed: there was significant interannual variability in naupliar 

 concentrations within the region around the Mississippi River delta; naupliar concentrations in the 

 upper 10 m decreased in the onshore-offshore direction in 2 of 4 comparisons; naupliar concentrations 

 in the upper 10 m differed regionally in 2 of 3 comparisons; and naupliar concentration was correlated 

 with chlorophyll concentration in 4 of 5 comparisons. 



Maximum concentrations of nauplii (number per m^) within a water column were 2-10 times 

 greater at stations influenced by the Mississippi River plumes than in the other 2 regions. This 

 condition in attributed to vertical stratification imparted to the water column by the inflowing low 

 salinity water from the Mississippi River. We conclude that the physical stratification provides a 

 mechanism for the establishment of high concentrations of nauplii that otherwise would not exist in 

 the winter months on the continental shelf. 



Microzooplankton are important diet items for 

 larval fish (Arthur 1976; Gamble et al. 1981; 

 Checkley 1982; Govoni et al. 1983; Houde and 

 Lovdal 1984; Stoecker and Govoni 1984), and gut 

 analyses indicate that copepod nauplii are fre- 

 quently the dominant prey form found in the lar- 

 vae of many fish species (Duka and Gordina 

 1973). Available concentrations of micro- 

 zooplankton are considered an important deter- 

 minant of larval survival rates in the ocean be- 

 cause this relationship has been demonstrated in 

 the laboratory (Laurence 1974; Houde 1978) and 

 because field studies have demonstrated a rela- 

 tionship between regions or periods of high mi- 

 croplankton and high larval abundance (Arthur 

 1977; Lasker 1978). Because survival is enhanced 

 by increased food availability, oceanographic 

 processes that result in increased concentration 

 or production of prey items are important. 



The gulf menhaden, Brevoortia patronus (Clu- 

 peiformes), supports the largest volume fishery in 

 the United States (U.S. Department of Commerce 

 1983). Spawning occurs in the wintertime, from 

 October to March, in continental shelf waters of 

 the northern Gulf of Mexcio (Fore 1970; Lewis 

 and Rothmayr 1981; Warlen 1988), primarily off 



iLouisiana Universities Marine Consortium, Chauvin, LA 

 70344. 



^Applied Oceanic Marine Laboratory, Ocean Coastal Divi- 

 sion, NOAA, 4301 Rickenbacker Causeway, Miami, FL 33149. 



of Mississippi and Alabama to the east of the Mis- 

 sissippi River delta, and ofi" of Louisiana to the 

 west of the delta. During February and December 

 1982, concentrations of menhaden larvae in the 

 region near the Mississippi River delta were 

 greater in plume waters than outside and were 

 much higher at the plume front (Govoni and 

 Hoss"^). Furthermore, gut contents of these larvae 

 indicated that nauplii were the most abundant 

 prey items. 



Much of the continental shelf water of the 

 northern and western Gulf of Mexico is vertically 

 unstratified during the winter, and winter is also 

 the season of minimum primary productivity in 

 these regions, as it is in shelf waters of the north- 

 east and southeast United States. However, 

 coastal waters and nearshore regions influenced 

 by freshwater inputs can be physically stratified 

 during winter when low salinity plumes disperse 

 over higher salinity shelf waters. We postulated 

 that shelf waters influenced by fresh water from 

 the Mississippi River plumes would be regions of 

 increased production and concentration of micro- 

 zooplankton. The purpose of this paper is to exam- 

 ine this hypothesis by describing the vertical and 

 horizontal distribution and abundance of copepod 

 nauplii in shelf waters of the northern Gulf of 



Manuscript accepted December 1987. 

 FISHERY BULLETIN: VOL. 86, NO. 2, 1987. 



^Govoni and Hoss. Unpubl data. Southeast Fisheries Center 

 Beaufort Laboratory, National Marine Fisheries Service, 

 NOAA, Beaufort, NC 28516. 



319 



