In species with both male-male competition and male mate choice, inferior males may make different mate choice decisions from superior males. Males of the intertidal hermit crab, Pagurus middendorffii, are known to conduct precopulatory guarding and to adjust the threshold for guarding according to social parameters, such as encounter rate with females, competitor size and sex ratio. Larger males are stronger in male-male competition during guarding in this species. We here tested whether male of P. middendorffii initially guarding a smaller female exchanged partners when the male encountered a larger receptive female, and whether large and small males chose potential mates on the basis of body size and/or time needed for guarding when a male simultaneously encounters two females. When a male guarding a smaller receptive female encountered a larger receptive female, the male assessed the larger female and exchanged his partner only in cases of a large difference in body size between the two females, suggesting that males of this species could choose their mates based on female quality even during guarding. When a male simultaneously encountered two receptive females, small males showed the prudent mate choice by balancing female traits between larger body size and shorter time until breeding, while large males showed preference for larger females. The distinct preference exhibited by males of different size classes is concluded to be an adaptive response to the size-dependent risk of losing the female during guarding.
We examine geographical differences in percentages of age-1 Pacific saury Cololabis saira with previous spawning experience collected from 143°E to 165°W during June and July of 2013 and 2014. Previous spawning experience of fish was determined using a new histological method involving Victoria blue (VB)-positive ovarian arterioles. We also compared the radius of the otolith annual ring (ROA), which indicates fish body size at the beginning of the breeding season, with the incidence of previous spawning experience. A generalized linear model was used that treats the occurrence of fish with VBpositive arterioles as a response variable, following the Bernoulli distribution of probability p i , where longitude, latitude, body length, ROA in age-1 fish, year (2013 or 2014), sea surface temperature, and days elapsed from a survey starting date, are used as fixed effect terms. An estimated regression coefficient of longitude was negative while that of ROA was positive, meaning that the probability of previous spawning increases with a latitudinal progression west, and with increased ROA. Our results suggest that differences in the percentage of previously spawned fish in different geographic areas are caused by differences in body length at the beginning of the breeding season.
Although the number of marine protected areas (MPAs) for stock management has increased, movements or differences in population structure of a target species between an MPA and surrounding fishing areas have rarely been considered in stock biomass estimations. We developed a surplus production model considering seasonal movements between two areas; the model was applied to Sea Ravens Hemitripterus villosus off Fukushima, where almost all fishing has been prohibited since the 2011 accident at the Fukushima Dai‐ichi Nuclear Power Plant. We predicted future biomass by using CPUE data from coastal gill‐net fishing and offshore bottom trawl fishing in 2000 to 2009. The model reflected the seasonal coastal–offshore movements of Sea Ravens well, and it predicted increasing Sea Raven biomass in both areas, which was validated by the CPUEs observed after 2010—including those for trial bottom trawl fishing that occurred within limited offshore areas after the accident. Our results indicate that the newly developed model incorporating seasonal movements of Sea Ravens is feasible and that the waters off Fukushima have effectively been serving as an MPA since the nuclear accident. We also demonstrated the model's applicability for estimating the optimal fishing effort and designing a new MPA for stock management that considers seasonal movements.
Received August 29, 2014; accepted May 4, 2015
A maximum sustainable yield (MSY) obtained by maintaining or restoring fish stock levels is a tangible benefit of ecosystem services. Snow crab (Chionoecetes opilio) off Tohoku, Japan has been managed by a total allowable catch since 1996, although the abundance has not increased. Surprisingly, there was no increase after 2011, when fishing pressure was greatly reduced because of the Great East Japan earthquake. This implies that some of the crab's biological characteristics, such as recruits, natural mortality coefficient (M) and terminal molting probabilities (p), might have changed. We developed “just another state‐space stock assessment model” to estimate the MSY of the snow crab off Tohoku considering interannual variations in M and p. The multimodel inference revealed that M increased from 0.2 in 1997 to 0.59 in 2018, although it did not vary according to instars, sex or terminal molt. The parameter p also increased by 1.34–2.46 times depending on the instar growth stages from 1997 to 2018. We estimated the MSYs in three scenarios, which changed drastically if M and p were set as they were in the past or at the current values estimated from this study. This result indicated that the MSY of snow crab would also vary with time based on their time‐varying biological characteristics.
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