An investigation of batch-spawning reproduction in captive yellowtail flounder, Pleuronectes ferrugineus

Manning, Anthony J. (1996) An investigation of batch-spawning reproduction in captive yellowtail flounder, Pleuronectes ferrugineus. Masters thesis, Memorial University of Newfoundland.

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Abstract

The batch-spawning reproduction of female yellowtail flounder, Pleuronectes ferrugineus, was examined in a captive setting over the spawning periods of 1994 and 1995. Individual female yellowtail were examined daily with the objectives of determining the inter-ovulatory period of this species, as well as, the annual egg production, mean batch fecundity and number of batches per female. Egg quality measures (fertilization and hatching rates) were also made on batches from females to quantify maternal differences and inter-batch variation within females in egg survival rates. -- A one-day inter-ovulatory period was predominant in this species and the frequency of this period increased in 1995 due to reductions in irregular ovulatory activity detected in certain females in 1994. The mean duration of spawning for individual females ranged between 31 to 48 days, over which time egg diameter and dry weight were observed to decrease within individual females. -- Female yellowtail flounder demonstrated considerable individual variation in seasonal egg production. In 1994, the mean female production was 550,000 eggs (7.9 x 10⁵ eggs kg⁻¹ spawning female), in 1995 this increased to 1,187,000 eggs (1.5 x 10⁶ eggs kg⁻¹ spawning female). The number of ovulations per female was high, a mean batch number of 14 was seen in 1994, rising to 22 in 1995. Small batch sizes (<100,000 eggs) dominated batch fecundity distributions, and the data suggested that females increase the number of ovulations rather than increase batch fecundity when egg production increases between years. -- Egg quality in yellowtail flounder was found to be subject to maternal differences. In addition, females demonstrated significant inter-batch variation in fertilization and hatching rates. An inter-annual increase in egg quality was observed, where mean fertilization rates rose from 38% to 60%. This increase was not due to slower over-ripening rates since spawning temperatures were not significantly different between years. The egg quality data also did not demonstrate any period within an individual female's spawning season in which the batches with the highest quality eggs were produced. A relationship was seen between gamete potential rates, determined by egg morphological characteristics, and fertilization rates. However, since individuals had their own separate relationships, gamete potential rates can only serve as a rough estimate of egg quality, where fertilization rate were generally 25% lower than gamete potential rates. Hatching rates were, overall, higher than fertilization rates and showed no relationship with fertilization rates. -- Investigations on over-ripening indicated that a daily examination protocol is required to avoid reductions in fertilization rate which were greatest during the first 24 hours after collection. Hatching rates declined less dramatically than fertilization rates with over-ripening. -- The inter-annual increase in the reproductive performance for captive yellowtail flounder may have been based upon reductions in stress from an additional year of acclimation to captive conditions or the introduction of a commercial feed in 1995. Either factor may have been responsible for the reduction of females with poor egg quality and the observed increases in egg production for spawning females.

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