FISHERY BULLETIN: VOL. 80, NO. 3 



boundary of the geographical range of soft-shell 

 clams (Manning 12 ), cold water temperatures 

 may, in some unexplained way, enhance the sur- 

 vival of a previous year's set. 



Manning and Dunnington (1956) showed that 

 Maryland clams grow at a rapid rate, achieving 

 legal size (2 in, 5.1 cm) at an age of 16 to 22 mo. 

 Hence clams spawned in the fall of one year 

 would enter the commercial fishery during the 

 spring 2 yr later. Extreme low water tempera- 

 tures generally occur in January or February of 

 each year and during some years coincide with 

 periods of high salinities. Thus, both variables in 

 the regression model could be exerting an effect 

 on juvenile clams from set to the age of 6-7 mo, 

 when they are approximately 0.5 cm (0.2 in) in 

 size. Low water temperature may delay move- 

 ment of predators into Maryland waters, permit- 

 ting juvenile clams to grow to a less vulnerable 

 size. High salinities during the juvenile life 

 stages could also favor growth and rapid matur- 

 ation of clams. 



The remaining four regressions are composed 

 of variables which are less readily explained. 

 The two terms entering the menhaden regres- 

 sion have 4 and 5 yr lags (Table 2). But menhaden 

 which make up Maryland landings are of ages 2 

 and 3, with almost no 4-yr-old fish taken (Merri- 

 ner 13 ). Thus, any causal mechanism suggested by 

 the regression would have to be a second genera- 

 tion response, remembering that menhaden do 

 not become sexually mature until ages 3 or 4. 

 Ep5 appeared as the most important predictor in 

 37% of the trials with CT4 following as a second- 

 ary predictor in 23% of those cases. Menhaden 

 catch is depicted in Figure 2. 



Juvenile bluefish use Chesapeake Bay as a nur- 

 sery area and there is the possibility that a dis- 

 tinct Chesapeake Bay stock of bluefish exists 

 (Kendall and Walford 1979). Bluefish, being a 

 marine species, are generally not found in low 

 salinity waters, and their distributions can be 

 well defined by salinity patterns (Lippson et al. 

 1980). Thus, the precipitation variable entering 

 the regression (Table 2) may reflect diminished 

 nursery habitat caused by high precipitation, re- 

 sulting in a decline of harvestable fish in future 



5000 



12 Manning, J. H. 1957. The Maryland soft-shell clam in- 

 dustry. Study Report 2, 25 p. Maryland Department of Re- 

 search and Education, Solomons, Md. 



13 J. V. Merriner, Chief, Division of Fisheries, Southeast 

 Fisheries Center Beaufort Laboratory, National Marine Fish- 

 eries Service, NOAA, Beaufort, NC 28516, pers. commun. Sep- 

 tember 1980. 



Figure 2.— Actual (solid line) and predicted (dotted line) 

 catches in metric tons of menhaden, 1946-76, based on the re- 

 gression model in Table 2. Environmental factors were an 

 episode of low daily precipitation (with a 5-yr time lag behind 

 the harvest figure), and cumulative high deviation in air tem- 

 perature (4-yr time lag) which had a negative effect. 



years. However, age composition of Maryland 

 bluefish catch is unknown, and the particular 

 lags in the regression are not readily explained. 

 Bluefish harvests are illustrated in Figure 3. 

 Ep5 appears as the primary predictor in 56% of 

 the trials run with Cs2 following in 23% of those 

 instances. It is noteworthy that the same variable 

 (Ep5) appears as the most useful predictor of 

 both menhaden and bluefish. Both species are 

 coastal spawners, and it is entirely possible that 



250 



1980 



Figure 3.— Predicted (dotted line) and recorded (solid line) 

 weights of bluefish landings in metric tons, 1947-76, based on 

 the regression model in Table 2. An episode of low daily pre- 

 cipitation (5-yr time lag), and cumulative low deviation in 

 salinity (2-yr time lag) were the environmental factors. 



616 



