Comparison on two different methods for assessment of population fertility of the Okhotsk herring on the data of standard roe survey

Abstract

The main part of the spawning grounds of herring in the northwestern Okhotsk Sea, from the Ayan Bay to the Eirineiskaya Bay, was surveyed on June 3–21, 2022 using standard methods of the roe diving survey. In order to rationalize the survey methodology, the herring population fertility (PF) was evaluated using two different approaches. The first is a standard method (described by Farkhutdinov, 2005) with calculation of the spawning ground areas and the roe density (RD) for each of them. The second method is based on the relationship between the total area of spawning grounds and PF detected previously by Dulenin and Didenko (2021). The spawning conditions for Okhotsk herring were abnormal in 2022. Just before the mass spawning, a strong storm had destroyed many spawning substrates and redistributed the spawning shoals of herring in the coastal zone. As the result, mean RD was extremely low – 1075318 eggs/m2 (long-term average value was 4170320 eggs/m2), but herring spawned in new areas, additionally to usual spawning grounds, so the total area of surveyed spawning grounds was rather large, as 38.68 km2. The population fertility calculated for this total area using the relationship between these indices was 182 . 1012 eggs that corresponded to the spawning stock number (SSN) of 6.513 . 109 ind. and the spawning stock biomass (SSB) of 1.823 . 106 t. These estimates match well with results of the stock assessment by other methods. On the contrary, unrealistic low assessments were provided using the routine method with calculation of RD on the annually surveyed spawning grounds (PF 51 . 1012 eggs, SSN 1.936 . 109 ind., SSB 0.542 . 106 t), because the spawning on destructed substrate could not represent adequately the state of the herring population during 2–3 years, until restoration of the algae belt. Besides, the standard method for data processing of the roe diving surveys is based on a number of non-formalized subjective assessments and assumptions that cannot ensure verification of the survey results. In opposite to this disadvantage, the calculation of PF by the area of spawning grounds is methodologically simple and allows to use remotely operated underwater vehicles, unmanned aerial vehicles and geographic information systems, with complete hardware and software documentation of survey materials. Thus, the PF evaluation using its relationship with the total area of spawning grounds is recommended for introduction as the main method of the Okhotsk herring stock assessment.