Year-class Indices Research Articles

Investigating the Applicability of Ichthyoplanktonic Indices in Better Understanding the Dynamics of the Northern Stock of the Population of Atlantic Hake Merluccius merluccius (L.)

Abundance indices are essential data for the application of stock assessment models to obtain fish abundance estimates. Abundance indices have usually been derived from fishery-dependent data, yet the increase in fisheries-independent surveys is now offering new opportunities for these calculations. In this study, we explored the usefulness of ichthyoplankton indices derived from scientific surveys in estimating spawning biomass. In addition, we also investigated whether the strength of the year–class of the commercial cohort of Atlantic hake, as a determinant, could be defined at an early life stage. We used samples collected during the triennial mackerel and horse mackerel egg surveys (MEGS), which cover the hake spawning area in the Bay of Biscay. The biomass indices were determined as the abundance of eggs in the early development stage (stage 1) when transformed into egg production (EP) from 1995 to 2019 in the months of March and April—which is considered a period of high spawning activity for hake in this area. Additionally, we built a metric for larval abundance and converted larval length into age. This was in addition to constructing a pre-recruit year-class index (YCI) while using the EVHOE bottom trawl abundance database for hake for the period of 1997 to 2016. The results of regression analysis of egg production and spawning stock biomass indicate that both parameters are significantly correlated (r = 0.76). By connecting the abundance of eggs and larvae in the adjoining stages, we are able to identify two periods of high mortality associated with the transition from “yolk-sac-first” to “feeding larvae” and “late larvae-YCI10”, but we were unable to discover when the strength of the recruitment year–class is determined. As such, it appears that for the northern stock of hake, recruitment is established in the late juvenile stages.

The Effect of Annual Water Level Management on Population Trends of White Crappie in Elk City Reservoir, Kansas

Abstract To improve a marginal white crappie (Pomoxis annularis) sport fishery in a 1,800 hectare Kansas reservoir, a management plan for water level manipulation was devised to increase the population. The essence of the plan was to increase the water area during the spawning season to stimulate natural reproduction, while flooded vegetation increased the amount of spawning habitat. The effect of this procedure was significantly (P < 0.05) related to an increase in the relative year-class strengths of white crappie during a 5-year period, as compared to year-class indices measured 5 years previously. However, the hectare-days of flooded vegetation during the spawning season were not significantly related to the increase in crappie numbers, but may have been overshadowed by environmental conditions that limited reproduction or recruitment of the white crappie. Fluctuations in water levels during the crappie spawning season had a significant (P < 0.05) but negative relationship on crappie year-class streng...