250 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



n=the number of stations considered 

 Cj=the average number of eggs spawned 



per day at the ith station^ 

 Wi= the weighting factor for space in 

 standard area (i.e., units of 10 m^ 

 of sea surface) 

 ti=the time factor which is eciual to 

 one-half the time from the preceding 

 occupancy of the station pUis one- 

 half the time to the succeeding 

 occupancy. 

 An annual estimate of abundance is obtained 

 by summing the monthly estimates for the entire 

 year. 



The eggs are identified and the number belong- 

 ing to each species is recorded by station. The 

 count for each station is adjusted so that all re- 

 sults are expressed as the number of eggs under a 

 standard area which is 10 square meters of sea 

 surface (Ahlstrom, 1953). This standardized haul 

 value (c'«) is the product of the raw count times a 

 standard haul factor, which is derived for each haul 

 by dividing 10 by the average volume of water 

 strained per meter of depth for the entire water 

 column. 



EFFECT OF TEMPERATURE ON LENGTH 

 OF INCUBATION PERIOD 



The length of the incubation period (dt) is 

 dependent on temperature of the water mass in 

 which the eggs are developing, and may be pre- 

 dicted if the temperature coefficient for the rate of 

 development is known. 



The effect of temperature on the rate of develop- 

 ment of jack mackerel eggs has been derived by 

 two methods. In the first method, the eggs were 

 taken from the sea shortly after they had been 

 spawned, placed in a fish egg incubator (see appen- 

 dix A) and observed at 4-hour intervals until they 

 hatched. The temperature of the water in the 

 incubator was maintained at about 14° C, the 

 temperature of the sea water from which the eggs 

 were collected. The observed hatching time was 

 108.5 hours. This experiment was repeated 1 year 

 later at 15° C, with an observed hatching time of 

 84 hours. 



The second method, which is indirect, was de- 

 veloped by Ahlstrom (1943) for the Pacific sardine 

 and was also successfully used by Gamulin and 



Hure (1955) for sardines in the Mediterranean 

 Sea. A series of arbitrarily chosen but precisely 

 defined morphological stages is selected. Such a 

 series of stages is described for jack mackerel in 

 appendix B. The jack mackerel eggs from station 

 samples are separated into stages and counted 

 (Farris, 1958: table 2, p. 7-11). 



Several successive days of spawning are indi- 

 cated by the stage frequency modes present in 

 each sample. A mode is interpreted as repre- 



1500 1800 



2100 2400 0300 

 TIME OF COLLECTION 



0600 0900 1200 



' a is derived as follows: Qi=cJ<ii where ci = the standard number of cgps 

 at the ith station: d, = the time interval in days from spawning to hatching. 



Figure 2. — Stage-I jack mackerel eggs collected in 1951 

 by hour of collection. All stage-I eggs collected at a 

 particular hour are expressed as a proportion of the total 

 stage-I eggs for the year. 



sen ting 1 day's spawn, inasmuch as spawning is 

 limited to a short period each night and the modes 

 represent stages separable by a day as determined 

 from incubation experiments. 



The time of day at which spawning takes place 

 was determined by plotting the relative abundance 

 of precleavage eggs (stage I) against time of col- 

 lection (fig. 2). The time the samples were 

 collected is given in a report of the South Pacific 

 Fishery Investigations (1952). These data showed 

 the time of maximum spawning for jack mackerel 

 was approximately midnight. 



The age of the earliest stage is computed by 

 subtracting the hour of collection from midnight. 

 The age of the second mode is estimated by adding 

 24 hours to the age of the first mode, and the age 



