Effects of Maternal Growth on Fecundity and Egg Quality of Wild and Captive Atka Mackerel

Abstract Trade-offs in energy allocation between growth and reproduction can result in variations in reproductive potential in fish with differing growth patterns. Spawning biomass is often used as a proxy for reproductive potential on the assumption that fecundity is directly proportional to body weight. We examined variations in the reproductive potential of Atka mackerel Pleurogrammus monopterygius by studying the effect of differential growth and condition patterns on fecundity, atresia, and egg energy. Fecundity and egg energy were determined for fish from two geographic areas, Seguam Pass and Amchitka Island, Alaska, and compared with those of fish held in captivity. These Atka mackerel showed distinct differences in growth and condition, with weight at length and length at age being the highest among captive fish, intermediate among fish from Seguam Pass, and lowest among fish from Amchitka Island. Realized fecundity showed that on average captive fish spawned seven batches, fish from Seguam Pass six batches, and fish from Amchitka Island five batches. For wild fish, potential and realized fecundity at length or age was significantly higher at Seguam Pass than at Amchitka Island, whereas the fecundity-at-weight relationship did not differ by area, suggesting that weight is a better predictor of fecundity than length or age. Atresia and batch fecundity by length or weight did not differ by area, suggesting that the variation in fecundity is better explained by the variation in batch number than by batch size. Oocyte dry weight was higher for captive fish than for wild fish, whereas batch order did not significantly affect oocyte dry weight. Increased potential fecundity, realized fecundity, and oocyte quality in Atka mackerel females were strongly related to body size, indicating that growth differences and maternal feeding success impact the fecundity and oocyte quality of Atka mackerel. Therefore, changes in growth and condition patterns need to be taken into account to accurately estimate the reproductive potential of this species.


Background
Atka mackerel are the most abundant groundfish in the Aleutian Islands (2009 adult biomass 390, 000 metric tons). They are distributed in dense aggregations in areas of strong currents from Kamchatka to the Gulf of Alaska along the Aleutian Island chain.
Adult Atka mackerel do not move much. Tagging studies show that 95% of adult Atka mackerel stay within their home range (20-50 nautical miles) (McDermott et al., 2005).

Wild fish collection
Atka mackerel were collected during NMFS research cruises on trawl vessels. Five females were randomly collected during each trawl haul. Females were lengthed, weighed and the otoliths and ovaries were extracted. For each female, one ovary was processed into histological slides with H&E stain for ovary stage determination. The other ovary was subsampled for fecundity analysis using the gravimetric method.

Captive fish collection:
The 5 captive females were collected from various locations in the Aleutian Islands during a NMFS cruise using bottom trawl gear. The fish were held in seawater tanks until arrival in Dutch Harbor from where they were transported in oxygenated coolers to the Alaska Sealife Center in Seward. They were transferred into live tanks and fed to satiation three times a week for the remainder of the study.

Objective
Examine how growth and condition affect realized fecundity and egg energy: 1. Estimate total fecundity, batch fecundity, and atresia from three populations with different growth patterns: Amchitka Island, Seguam Pass, and fish held in captivity.
2. Estimate the egg energy content from wild fish and compare to it to fish held in captivity

Fecundity estimation for wild fish
Total fecundity, batch fecundity, and atresia were determined with the gravimetric method as described in McDermott 2005.
Total potential fecundity: all oocytes in vitellogenic stage and above were counted from pre-spawning females.
Batch fecundity: oocytes in late hydration stage were counted.
Realized fecundity: estimates of atresia at length were subtracted from estimates of potential fecundity at length.

Fecundity estimation for captive fish
In 2005 the five captive females spawned a total of 37 fertilized batches. Egg masses were collected shortly after spawning by divers. Parentages of egg masses were successfully determined with genetic analysis.
Batch fecundity: The number of eggs in each egg mass was determined with the gravimetric method.
Realized fecundity: The number of eggs from all egg masses spawned per female per season were summed.

Egg energy and egg dry weight
Egg energy of 30 wild caught females and five captive fish was determined using a semi-micro bomb calorimeter. Egg dry weight was strongly correlated with egg energy and was used as a proxy to compare wild and captive fish.

Conclusions:
Atka mackerel fecundity is best predicted by weight not length. Fecundity differed significantly for areas with different growth patterns. Egg quality was significantly higher for large captive fish than for smaller wild fish. In order to correctly estimate the reproductive output of Atka mackerel populations, it is important to track fish growth and condition on a local scale.
Fat big fish produce more and higher quality eggs.

Life history
Atka mackerel are semi-pelagic and mostly occur at a depth of 100-200m.
During the spawning season from July through October, males establish nesting sites where they actively guard the nests.
Atka mackerel females are batch spawners and spawn on average four to five batches of eggs in rock crevices or among stones. The eggs are fairly large (1-2 mm diameter) and will be glued together in tight egg-masses that attach to the substrate (McDermott et. al. 2007).