LImburg et aL: Growth, mortality, and recruitment of larval Morone spp. 



81 



tion rates and are at lower risk of exposure to subop- 

 timal or lethal temperatures. 



Do these temperature and food conditions trans- 

 late into differential survival conditions for larval 

 Morone spp. in the Hudson River? We addressed this 

 question using methods of otolith age and growth- 

 rate reconstruction. Specifically, we tested if periods 

 of high food availability (zooplankton bloom condi- 

 tions) were also periods of high larval growth rates. 

 Further, by analyzing the otoliths of juveniles of the 

 same year class (i.e. successful recruits from the lar- 

 val stage), we also assessed whether or not fish that 

 survived the larval period had high larval growth 

 rates and hatching dates associated with the zoop- 

 lankton bloom. 



Studies of Morone saxatilis in Chesapeake Bay 

 tributaries have used both interannual variations in 

 larval abundance (Uphoff, 1989) and cohort analy- 

 sis methods (Rutherford and Houde, 1995; Secor and 

 Houde, 1995; Rutherford et al., 1997) to estabUsh 

 that rapid drops in water temperature below 12°C 

 virtually eliminate larval cohorts. These studies con- 

 cluded that the frequency of low temperature events 

 can be an important factor in the recruitment of 

 striped bass in those systems. In contrast. Pace et 

 al. (1993) found no evidence that temperature af- 

 fected interannual variation of Morone spp. recruit- 

 ment in the Hudson River, although Dey ( 1981) pre- 

 sented evidence for temperature-induced mortality 

 in Hudson River striped bass in 1976 (temperature 

 dropped suddenly from 15° to 12°C in late May). Thus 

 temperature effects, although not detectable among 

 years, might still manifest themselves as seasonal 

 variability. In this paper we examine the 1994 year 

 classes of striped bass and white perch, including an 

 appraisal of the relation of zooplankton abundance 

 and temperature to growth and mortality of indi- 

 vidual cohorts. With this information, we can make 

 tentative comparisons between analyses for the 

 Hudson River and Chesapeake Bay. 



Materials and methods 



The Hudson River, a partially stratified estuarine- 

 river system in New York State, is tidal up to the 

 Green Island dam, 245 river kilometers (rkm) from 

 the mouth. All life stages of white perch are found 

 throughout the oligohaline and freshwater sections 

 of the estuary (Klauda et al., 1988). Striped bass 

 adults migrate into the estuary in spring to spawn. 

 Larvae are found in both freshwater and oligohaline 

 sections (Boreman and Klauda, 1988). Juveniles 

 spread throughout the estuary and move seaward 

 during late summer and fall (Dovel, 1992). 



Larval collections and preparation 



Field collections of larval Morone spp. were under- 

 taken in the spring and early summer of 1994. Sam- 

 pling details are described in Limburg et al. (1997). 

 Briefly, three sites, in the upper (Kingston, rkm 148), 

 middle (New Hamburg, rkm 105), and lower 

 (Haverstraw Bay, rkm 65-70) portions of the estu- 

 ary were sampled between 18 May and 6 July; all 

 sites overlapped with spawning areas. Sampling fre- 

 quency was designed to examine larval and zooplank- 

 ton d3Tiamics before, during, and after the intensive 

 bloom of the cladoceran zooplankton Bosmina freyi 

 i=Bosmina longirostris with older nomenclature). 

 Fish were collected during the daytime with 5-min 

 oblique tows of 0.5-m diameter bongo nets (500-mm 

 mesh). Five replicate tows were conducted at each 

 site on each date. Samples were preserved in 100% 

 ethanol which, when diluted with the sample, 

 reached a final concentration of no less than 75%. 

 Care was taken not to over-dilute the samples below 

 this concentration because of risk of otolith erosion 

 (Brothers, 1987). 



In the laboratory, fish were sorted by family and 

 stored in fresh 100% ethanol. Three replicates were 

 chosen at random from the five available at indi- 

 vidual stations for each sampling date, and up to 

 twenty Morone individuals were arbitrarily selected 

 from each replicate; more replicates were used when 

 larval abundances were low. The larvae were re- 

 hydrated, soaked for 15 min in sodium borate buffer 

 solution (30% saturated sodium borate), and then 

 cleared by adding a small quantity of trypsin to the 

 buffer solution. Morone larvae were measured to 0.1 

 mm notochord length (NL) and identified to species 

 by a combination of internal and external charac- 

 ters (Limburg et al., 1997). Lengths were not cor- 

 rected for preservation shrinkage, but rehydration 

 and clearing fully relaxed constricted musculature 

 so that lengths were readily measured. Otoliths were 

 visible under 25x magnification on a stereomicro- 

 scope. Fine insect pins, the tips of which were sharp- 

 ened further, were used to dissect the sagittal 

 otoliths. Otoliths of fish less than 5 mm NL were 

 cleaned in deionized water and placed directly in 

 mineral oil for later viewing. Following the sugges- 

 tion of Rutherford (1992), otoliths of larger (>5 mm 

 NL) fish were embedded in Spurr's epoxy, then 

 ground and polished with a series of increasingly fine 

 grinding papers (down to 3 mm). 



Juvenile fish collection and preparation 



In order to make comparisons between recruited ju- 

 veniles and larvae, we obtained a total of 218 juve- 



