156 



Fishery Bulletin 91(1). 1993 



Temporal distribution 



Results 



Spanish mackerel occur in Chesapeake Bay from late 

 April to early October. They first appeared in the 

 catches on 15 May 1989 (Fig. 2) when three fish were 

 taken off Lynnhaven, on 26 April 1990 (Fig. 3) when 

 two fish were taken there, and, anecdotally according 

 to those fishermen, on about 10 May 1991. Though 

 fishing continued well afterwards, the last catches were 

 on 3 October 1988 and 2 October 1989, dates when we 

 recorded only a few individuals at the lower York River 

 and Lynnhaven fisheries, respectively. 



Peak abundance of Spanish mackerel in Chesapeake 

 Bay occurs from early or mid-June through mid- 

 August, based on pound-net records. After the first 

 appearance in 1989, catches at Lynnhaven rapidly rose 

 to high levels in early to mid-June and remained high 

 through mid- to late August (Fig. 2). Combined catches 

 at Reedville, the lower York River, and the Eastern 

 Shore showed the same pattern. Comparatively few 

 fish were captured in any area after late August or 

 early September in that year. After fish appeared in 

 late April 1990, catches at Lynnhaven remained low 

 through at least early May (Fig. 3) when our observa- 

 tions temporarily ceased. Catches were large at 

 Lynnhaven by early to mid-June when observations 

 were made again. 



Our interpretation of temporal patterns in Spanish 

 mackerel abundance is supported by significance tests 

 that evaluate the null hypothesis of no difference in 

 catch between months within locations. Significant dif- 

 ferences in catch were found between months at each 

 location in 1989 (Table 1). Catches were significantly 

 higher at each location in summer months (June, July, 

 August) than in early spring (March, April) or late fall 

 (October, November) (Table 2). As typically occurs with 

 multiple comparisons tests, intermediate-size catches 

 in May and September were or were not significantly 

 different from adjacent periods of higher or lower catch; 

 the trend of increasing abundance to midsummer and 

 decreasing abundance into fall is the most important 

 feature here. 



Spanish mackerel abundance during the season fol- 

 lows a unimodal pattern. Catches in 1989 at Lynnhaven 

 especially, along the Eastern Shore, and off the lower 

 York River show a roughly bell-shaped distribution 

 (Fig. 2). Catches may be bimodal at Reedville near the 

 upbay margin of the range. 



Spanish mackerel occur in Chesapeake Bay when 

 water temperatures near the Bay mouth exceed about 

 17° C. The first fish were taken at Lynnhaven when 

 temperatures had risen to 17° C in 1989 (Fig. 2), and, 

 in 1990, 19° C after a period of rapid temperature in- 



crease in late April (Fig. 3). Large catches began in 

 late May in 1989, soon after temperatures rose to 20° C 

 (Fig. 2), and catches remained large through midsum- 

 mer at 21-27°C. The last fish were taken at Lynnhaven 

 on 2 October 1989 when temperatures decreased and 

 remained below 21°C. 



Discussion 



Our data on the temporal occurrence of Spanish mack- 

 erel in Chesapeake Bay agree with Earll (1883) and 

 Hildebrand & Schroeder (1928) in that (1) the overall 

 period of occurrence in this species is generally mid- 

 May through early October, (2) peak abundance is early 

 June through mid-August or mid-September, (3) the 

 last records of catches are all in early October, and 

 (4) the initial records of appearance are generally in 

 mid-May (10, 15 May in our records; 12, 20 May in 

 previous records), though fish may appear consider- 

 ably earlier (26 April, our records). Our late- April record 

 may reflect the early, rapid temperature increase that 

 occurred in 1990. The period when Spanish mackerel 

 occur in Chesapeake Bay is shorter than their late- 

 April to early-November distribution off North Caro- 

 lina (Earll 1883, Smith 1907, Roelofs 1951) but is some- 

 what longer than their distribution off New York and 

 New Jersey, variously reported as late May or late 

 July to late September-early October (Earll 1883, Bean 

 1903, Nichols & Breder 1926). The bell-shaped distri- 

 bution of catches that we, and apparently Hildebrand 

 & Schroeder (1928), observed for Chesapeake Bay dif- 

 fers from a bimodal distribution (i.e., peak abundance 

 in spring and fall) reported for North Carolina (Smith 

 1907, Hildebrand & Cable 1938, Roelofs 1951). Pre- 

 sumably, this reflects a north-south migration by part 

 of the population(s) through North Carolina waters in 

 spring and fall, in contrast to a summer residence in 

 the Chesapeake. 



Munro (1943) reported that the genus Scom- 

 beromorus is subtropical and tropical in distribution, 

 the optimum range of all species being within the 20° C 

 ocean isotherm in summer. Our findings agree, in that 

 Spanish mackerel initially appear in Chesapeake Bay 

 at temperatures of about 17-19° C and become abun- 

 dant at about 20° C. Other reports also support that 

 value (Earll 1883, Manooch 1984, Goode 1888). Perret 

 et al. (1971) captured one fish at 10°C, but that ap- 

 pears unusual. Beaumariage (1970) related the 20° C 

 ocean isotherm to Spanish mackerel distribution and 

 suggested Long Island would be near their northern 

 limit in August. Indeed, they are uncommon off Mas- 

 sachusetts (Nichols & Breder 1926, Bigelow & 

 Schroeder 1953). The time-period when temperatures 

 are above 20° C decreases with increasing latitude, and 



