Each year, the IPHC Fishery-Independent Setline Survey (FISS) recollects biological data on the maturity of female Pacific halibut that are used in the stock assessment. In particular, female maturity schedule is used to estimate spawning stock biomass. Currently used estimates of maturity-at-age indicate that the age at which 50% of female Pacific halibut are sexually mature is 11.6 years in average. However, female maturity schedules have not been revised in recent years and may be outdated. In addition, the currently used macroscopic visual criteria used to score female maturity in the field have an undetermined level of uncertainty and need to be contrasted with more accurate microscopic (i.e. histological) criteria. In order to address these issues, the IPHC Secretariat has conducted for the first time a thorough histological assessment of the temporal progression of female developmental stages and reproductive phases throughout an entire reproductive cycle. The outcomes of these studies have paved the way for upcoming studies to update and improve the accuracy of maturity schedules based on histological-based data and also to guide efforts towards assessing fecundity in Pacific halibut.
The IPHC Secretariat has completed the first detailed examination of temporal changes in female ovarian developmental stages, reproductive phases, and biological indicators of Pacific halibut reproductive development. The results obtained by ovarian histological examination indicate that female Pacific halibut follow an annual reproductive cycle involving a clear progression of female developmental stages towards spawning within a single year. In addition, histological analyses of oocyte development patterns in females at different developmental stages indicate that female Pacific halibut undergo group-synchronous ovarian development, are batch spawners and that have determinate fecundity. The results of these studies have been published in the journals Journal of Fish Biology: https://doi.org/10.1111/jfb.14551, and Frontiers in Marine Science: https://doi.org/10.3389/fmars.2022.801759.
Overall, these recent results provide foundational information for upcoming studies aimed at updating maturity ogives by histological assessment and at investigating fecundity in Pacific halibut. One of the most important results obtained show that the period of time when gonad samples can be collected in the FISS (June-August) is an appropriate temporal window during which Pacific halibut females that are developing towards the spawning capable reproductive phase and, therefore, considered mature for stock assessment purposes, can be identified.
In 2022, the IPHC Secretariat initiated studies to revise maturity schedules in all four biological regions through histological (i.e. microscopic) characterization of maturity. For that purpose, the IPHC Secretariat collected ovarian samples for histology during the 2022 FISS. The FISS sampling resulted in a total of 1,023 ovarian samples collected coastwide for histological analysis, with 440 ovarian samples from Biological Region 2, 351 samples from Biological Region 3, 181 from Biological Region 4, and 51 samples from Biological Region 4B. Ovarian samples have been processed for histology and IPHC Secretariat are currently finalizing scoring samples for maturity using histological maturity classifications as previously described in Fish et al. (2020, 2022). Following this maturity classification criteria, all sampled Pacific halibut females will be assigned to either the mature or immature categories. IPHC Secretariat continued to collect ovarian samples in 2023 on the FISS. This will allow us to investigate both spatial and temporal differences in female Pacific halibut maturity. Due to the reduction in FISS design for 2023, sampling efforts only took place in IPHC Biological Regions 2 and 3. A total of 1,110 ovarian samples were collected for histological analysis, with 403 samples from Biological Region 2, and 707 samples from Biological Region 3. Maturity ogives will be generated by age and length at a coastwide scale as well as at a biological region scale.
An important existing knowledge gap regarding the reproductive biology of Pacific halibut is the current lack of understanding of fecundity-at-age and fecundity-at-size. Information on these two parameters could be used to replace spawning biomass with egg output as the metric of reproductive capability in the stock assessment and management reference points. The IPHC Secretariat has investigated different available methods for fecundity determinations and, based on the current literature and recommendations from experts in the field, the auto-diametric method was selected as the method of choice (Witthames et al., 2009. Fish. Bul. 107:148-164). For this purpose, the IPHC Secretariat collected gonad samples for fecundity estimations during the 2023 FISS. The IPHC Secretariat targeted Biological Region 3 for this collection, with a total of 456 gonad samples collected.
Figure 1. Progression of female developmental stages during an annual reproductive cycle in relation to the summer months when the IPHC fishery-independent setline survey (FISS) takes place. PGpn (primary growth stage), CA (cortical alveoli stage) Vtg1 (vitellogenic stage 1), Vtg2 (vitellogenic stage 2), Vtg3 (vitellogenic stage 3), GVM (germinal vesicle migration stage), PO (periovulatory stage), PS (post-spawning stage). Modified from Fish et al., 2022.
Fish, T., Wolf, N., Harris, B.P., Planas, J.V. A comprehensive description of oocyte developmental stages in Pacific halibut, Hippoglossus stenolepis. Journal of Fish Biology. 2020. 97: 1880-1885. doi: https://doi.org/10.1111/jfb.14551
Fish, T., Wolf, N., Smeltz, T.S., Harris, B.P., Planas, J.V. Reproductive biology of female Pacific halibut (Hippoglossus stenolepis) in the Gulf of Alaska. Frontiers in Marine Science. 2022. 9: 801759. doi: https://doi.org/10.3389/fmars.2022.801759
Witthames, P.R., Greenwood, L.N., Thorsen, A., Dominguez, R.,Murua,H., Korta,M., Saborido-Rey, F., Kjesbu, O.S., 2009. Advances in methods for determining fecundity: application of the new methods to some marine fishes. Fishery Bulletin 107, 148–164.