Morphometric separation of annual cohorts within mid-Atlantic bluefish, Pomatomus saltatrix, using discriminant function analysis*

Larval and tagging stud- ies neither support nor refute the ex- istence of two mid-Atlantic stocks of bluefish, Pomatomus saltatrix, one that spawns just south of Hatteras, North Carolina, in the spring, and the other off New England in the summer. Sub- sequent studies have suggested either a continuous spawning with two sur- vival events, or two distinct spawning events, one in spring south of Hatteras, NC, and a second in mid-summer from Long Island to New England. Genetic studies have supported a single stock because they show the existence of a single gene pool. A stepwise linear dis- criminant function analysis (DFA) of morphometrics of "spring-spawned" yearling (200-400 mm) and older (>400 mm) bluefish did not classify fish by probable spawning cohort. It did, how- ever separate the yearling fish (200- 400 mm) by year class rather than geo- graphic or seasonal spawning. Older fish (>400 mm) showed less separation because multiple (2-10) year classes were present. The DNA studies have revealed genetic homogeneity among these fish. This finding suggests that the morphological characteristics are phenotypically plastic and are influ- enced each year by the physical envi- ronment during spawning and the early juvenile stages. The question of bluefish (Pomatomus saltatrix) stock composition along the east coast of the United States has been of considerable interest to fisheries scientists for 30 years and has recently (since 1986) become the focus of discussion by management agencies, primarily the Atlantic States Marine Fisheries Commis- sion (ASMFC) and Mid-Atlantic Fisheries Management Council (MAFMC) (Anonymous, 1989). The problems of managing fisheries when the stock composition and boundaries are unclear have ham- pered effective management of weakfish, Cynoscion regalis, sum- mer flounder, Paralichthys den- tatus, surf clam, Spisula solidis- sima, and striped bass, Morone saxatilis by the ASMFC or MAFMC (or both) because current manage- ment practices manage by unit (ge- netic) stock. Interstate manage- ment is further hampered when a unit stock exhibits differential spa- tial reproduction and migration pat- terns (e.g. weakfish, Scoles, 1990). The stock structure of the blue- fish (Pomatomus saltatrix) in the western mid-Atlantic region is not well understood. Lund (1961) re- corded meristic counts of gill rak- ers along the first branchial arch of young fish and suggested that six separate stocks of bluefish occur along the western north Atlantic coast of the United States. He later suggested the occurrence of several races in this region on the basis of results of two mark-recapture stud- ies (Beaumariage and Wittich, 1966; Lund and Maltezos, 1970). These results were not supportive of Lund's original six stock concept because they suggested that there might be a "Florida" or "South At- lantic Bight" and a "northern" or "mid-Atlantic Bight" stock. Subse- quent analyses of temporal and spa- tial distributions of bluefish in the ichthyoplankton, and spawning times, suggested two north Atlan- tic stocks (Norcross et al, 1974; Kendall and Walford, 1979), in ad- dition to a south Atlantic or Florida Stock (Collins and Stender, 1987). Chiarella and Conover (1990), on the other hand, using back calcula- tions of scales, demonstrated that most spawning in the north mid- Atlantic Bight (MAB ) occurs dur- ing mid-summer (July) and is com- posed of spring-spawned fish, both of which suggest a single stock. They also found from back calcula- tions that most yearling fish (260 mm) collected in waters along Long