Terwilliger and Munroe: Age, growth, longevity, and mortality of Symphurus plagiusa 



355 



C. semilaevis from Chinese waters, a strong sexual 

 dichotomy in longevity was apparent in the popula- 

 tion studied; females reached 14 years (most 2-8 

 years), whereas males in this population attained a 

 maximum age of only 5 years (Meng and Ren, 1988). 

 By comparison, longevity estimates for a variety of 

 other flatfishes occurring in temperate latitudes and 

 from families other than the Cynoglossidae range 

 from 6 to 30 years (Devoid, 1942; Arora, 1951; Pitt, 

 1967; Lux and Nichy, 1969; Lux, 1970. 1973; Koski, 

 1978; Wada, 1970; Zoutendyk, 1974; Smith and 

 Daiber, 1977). 



As stated earlier, the population of S. plagiusa in 

 Chesapeake Bay represents the northernmost loca- 

 tion of a major population of this species (Munroe, 

 1998). In the western North Atlantic, variability in 

 abiotic conditions increases from southern to north- 

 ern latitudes (Parr, 1933; Miller et al., 1991). Abiotic 

 variability also increases in environments located 

 closer to the shoreline (Russell-Hunter, 1970; Miller 

 et al., 1991 ). Fluctuations in abiotic factors, especially 

 seasonal extremes of temperature during winter, may 

 contribute to the high level of total instantaneous 

 mortality measured for blackcheek tonguefishes 

 within Chesapeake Bay. Bottom trawls conducted in 

 this system during late winter and early spring of 

 some years ( 1996, for example ) catch numerous dead 

 and moribund tonguefishes (Geer'^), apparently due 

 to stresses associated with extremely cold water tem- 

 peratures (ca. 2°C. in 1996). More extreme winter 

 temperatures and longer duration of cold tempera- 

 tures may be a factor limiting the establishment of 

 populations of blackcheek tonguefishes in more 

 northern estuaries, where this species is known only 

 by the occurrence of newly settled juveniles. In coastal 

 estuaries throughout its U.S. geographic range, 

 blackcheek tonguefishes appear to be sensitive to ex- 

 treme winter temperatures, because winter mortali- 

 ties have also been reported for this species in estu- 

 aries even as far south as those in Texas (McEach- 

 ron et al., 1994). In addition to causing death out- 

 right, stresses associated with extreme winter tempera- 

 tures may also render weakened tonguefishes easier 

 prey to predators and thus further contribute to win- 

 ter mortalities. 



Mortality estimates for the population of black- 

 cheek tonguefishes residing in Chesapeake Bay may 

 also be inflated due to emigration of larger individu- 

 als out of the Bay and onto the nearby continental 

 shelf Hildebrand and Cable ( 1930) earlier had sug- 

 gested that adult blackcheek tonguefishes in North 

 Carolina estuaries undergo a seaward migration 



during May-August. An autumn emigration of 

 blackcheek tonguefishes from Chesapeake Bay to the 

 nearby shelf region just outside of the Bay may be 

 reflected in the significantly larger catches (mean 

 no./tow) of blackcheek tonguefishes reported (VIMS*') 

 in this region during fall sampling ( 1987, 75.4/tow; 

 1988, 25.7/tow) compared with springtime catches 

 (1987, 11.0/tow; 1988, 4.37/tow; 1989, 2.2/tow). Data 

 supplied in Wenner and Sedberry (1989) also sup- 

 port the hypothesis that there is a seasonal migra- 

 tion of larger tonguefishes out of estuaries and onto 

 the coastal shelf off South Carolina. They reported 

 that this species was widespread throughout the 

 survey area but noted that significant seasonal dif- 

 ferences were apparent in the frequency of occurrence 

 and abundance of S. plagiusa in coastal habitats. 

 This species occurred less frequently during winter 

 (48*^ of all tows; 566 individuals) than in spring (85% 

 of tows; 1,460 individuals), summer (76% of tows; 

 798 individuals), or fall (94% of tows; 5962 individu- 

 als). Catches (mean no./tow) of this species were low- 

 est in summer (8/tow) and winter (9/tow), higher in 

 spring (2 1/tow), and highest during the fall (62/tow), 

 when the largest fish (up to 210 mm TL) were taken. 

 Although Wenner and Sedberry (and other studies) 

 have reported that it is usually only larger-size 

 blackcheek tonguefishes that are captured on the 

 inner shelf region, no studies have verified any sea- 

 sonal or spawning movements of blackcheek 

 tonguefishes out of the estuaries, or the amount of 

 exchange of individuals within populations between 

 estuarine embayments and adjacent neritic habitats 

 on the inner shelf It is also unknown whether all 

 larger individuals undergo an ontogenetic migration 

 out of estuaries onto the nearby shelf; what portion 

 of a population residing in estuaries moves out onto 

 the shelf at any given age; what the estuarine resi- 

 dency time is for larger tonguefishes, especially in 

 such larger and deeper estuaries as the Chesapeake 

 Bay which may be utilized by larger tonguefishes for 

 longer periods of time; or even if further exchange of 

 individuals between these two environments occurs 

 once an individual has emigrated beyond the estua- 

 rine boundary. Analysis of catch curve data (Fig. 10) 

 may also lend insight on inflated mortality estimates. 

 Although the bimodality of the catch curve data in- 

 fers possible variability in year-class strength that 

 invalidates its use for mortality estimations ( Robson 

 and Chapman, 1961; Everhart and Youngs, 1981), 

 the data suggest that a seventh year class may ex- 

 ist, possibly on the nearby shelf region. If so, mortal- 

 ity estimates based on a maximum age of 5+ years 



^Geer.P. J. 1996. VIMS, Department of Fisheries, Gloucester 

 Point, VA 23062. Personal commun. 



^ VIMS, Department of Fisheries, Gloucester Point, VA 

 23062. Unpubl. data. 



