Abstract
The collection and presentation of accurate reproductive data from wild fish has historically been somewhat problematic, especially for serially spawning species. Therefore, the aim of the current study was to develop a novel method of assessing female spawning status that is robust to variation in oocyte dynamics between specimens. Atlantic cod (Barents Sea stock) were used to develop the new ‘ultrametric’ method, that is based on the progressive depletion of the vitellogenic oocyte pool relative to the rather constant previtellogenic oocyte (PVO) pool. Fish were subsequently partitioned into one of four categories that accurately reflected changes in their oocyte size frequency distribution characteristics and gonadosomatic index throughout spawning. The ultrametric method overcomes difficulties associated with presence of bimodal oocyte distributions, oocyte tails, lack of clear hiatus region, and presence of free ova, and can be implemented at a single sampling point. Much of the workflow is fully automated, and the technique may circumvent the need for histological analysis depending on the desired outcome. The ultrametric method differs from the traditional autodiametric method in that PVOs can be separated by ultrasonication and then enumerated, and ovarian homogeneity is not a mandatory requirement per se. The method is designed for determinate spawners but might be extended to include indeterminate spawners.
Highlights
Calibration curve requires ovarian homogeneity, which is typically seen in pre-spawners but not in spawners, where ovulated oocytes congregate in the ovarian lumen before egg release
Lowest proportion of previtellogenic oocyte (PVO) to 250–850 μm range oocytes, this oocyte ratio category (ORC) contains the highest proportion of VOs
In order to do this, 72 G. morhua belonging to the Barents Sea cod stock were used as the test species to develop a new ranking system (ORC), that is based on progressive depletion of the VO pool
Summary
Calibration curve requires ovarian homogeneity, which is typically seen in pre-spawners but not in spawners, where ovulated oocytes congregate in the ovarian lumen before egg release (e.g. sole, Solea solea[14]). Unlike experimental or aquaculture studies, reproduction assessment methods in the field must be compatible with on-going stock survey programs or sequential processing of landings that often happen within a few days. This utilisation of discreet sampling complicates tracking of gonad dynamics, especially during the intense, often relative short spawning season. We combined advanced image analysis techniques with histology to get an in-depth understanding of oocyte development patterns, regarding VOs and for the much smaller PVOs, and used this data to develop a ranking system that relies on progressive depletion of the VO pool throughout spawning This ranking system is named the ultrametric method where ‘metric’ refers to automated whole-mount measurements as for the autodiametric method, and ‘ultra’ reflects the incorporation of ultra-small oocytes (PVOs) into the calculation, and the use of ultrasonication to separate the oocytes prior to their staining and measurement. We addressed three main hypotheses as a means of validating the ultrametric method: (1) Post-ovulatory follicles (POFs) would accumulate throughout spawning due to persistence of POFs in cold-temperate environments[17]; (2) As cod is a capital breeder[18], body condition index should decline throughout spawning, and (3) The gonadosomatic index (GSI) is expected to drop markedly during the course of spawning[19]
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