Comparison between surface-reading and cross-section methods using sagittal otolith for age determination of the marbled sole Pseudopleuronectes yokohamae

To find an appropriate method for age determination in the marbled sole Pseudopleuronectes yokohamae in Tokyo Bay, Japan, sagittal otoliths of 1,343 individuals were observed by surface-reading and cross-section methods and the results were compared. Opaque zones occurred once a year and were regarded as annuli in both methods. The surface-reading method sometimes provided a lower count of the number of annuli than the cross-section method, and the frequency of this discrepancy was highest in older fish (males above 5 years, females above 4 years). The oldest female fish was estimated to be age 10 years by the cross-section method but 8 years by the surface-reading method. The cross-section method could provide a more accurate estimate of age and is therefore likely to be indispensable to estimations of longevity. In contrast, the surface-reading method is superior in terms of cost and time efficiency but is likely to underestimate the ages of older fish. However, growth equations based on age estimated by the surface-reading method were sufficiently accurate if males ?5 years and females ?4 years were combined as specific, single age groups of 5+ and 4+, respectively

Comparison of the nutritional transition date distributions of marbled sole larvae and juveniles in Hakodate Bay, Hokkaido

To examine survival processes in marbled sole Pseudopleuronectes yokohamae larvae, nutritional-transition-date distributions (NTDDs) were compared between larvae and large juveniles (LJs) in Hakodate Bay from 2001 to 2003. NTDDs were used instead of hatch-date distributions because the onset of increment formation coincides with the timing of the transition from endogenous to exogenous nutritional sources for marbled sole. LJs were defined as being large enough to be safe from predation by the sand shrimp Crangon uritai. In 2001 and 2002, the ranges of the NTDDs of LJs largely overlapped with those of the larvae. However, in 2003, the early cohort (those whose otolith rings formed before 17 March) disappeared from the NTDDs during and after April, and the NTDDs of LJs were heavily biased toward a later period than those of the larvae. In 2003, Oyashio Coastal Water (< 3 ℃ and 33.0 salinity) unexpectedly intruded into the bay in late March and lowered the water temperature from 6.3 ℃ on 6 March to 4.1 ℃ on 25 March. One possible cause of mortality in the early cohort of 2003 might be feeding failure caused by the extremely low water temperature.マコガレイ仔魚の生残過程を調べるため、耳石日周輪解析を行い、内部栄養から外部栄養への転換日分布（以下：NTDD）を仔魚と稚魚との間で比較した。2001～2002年では、仔魚と稚魚のNTDDの範囲はおおむね重複していた。一方、2003年では3月17日以前に栄養転換した個体が4月以降のNTDDから消失していた。2003年には3月下旬に沿岸親潮が函館湾内へ流入し、湾内の水温が著しく低下した。この著しい低水温により3月17日以前に栄養転換した仔魚のほとんどが死亡した可能性が示唆された