Shellfish Biology

Novigradsko područje karakterizira polusredozemna klimatska zona s nešto većim dnevnim i godišnjim kolebanjima temperatura. Prosječna godišnja temperatura je izmeñu 10- 15ºC, dok se ekstremne vrijednosti temperatura kreću od -10 do +35ºC. Najhladniji su mjeseci siječanj i veljača sa prosječnih 5,5ºC, a najtopliji srpanj i kolovoz sa prosječnih 23-24ºC. Novigradsko more je zaljev usječen duboko u kopno. Predstavlja izolirani, izrazito razveden i složen hidrogeomorfoški sustav. U središnjem dijelu sjeveroistočnog ruba Novigradskog mora je zaljev ušća rijeke Zrmanje. Navedeni akvatorij po svojim značajkama predstavlja estaurij, vrlo dinamičan sustav nešto nižeg biodiverziteta u usporedbi s drugim graničnim morskim i slatkovodnim sustavima. Za njih su značajna izrazita prostorna i vremenska kolebanja hidrografskih čimbenika.
Nastala su u postpleistocenu, prodiranjem mora u ove kotline s neogenskim naslagama. Novigradsko je more s prosječnom dubinom u središnjem dijelu od 28 m, svojom površinom od 28,65 km² (dužina 11 km, širina 5 km) i vodenom masom od 0,5 km³, veće od Karinskog mora, koje obuhvaća površinu od 5,4 km² (dužina 3,8 km, širina 2,4 km) i volumen od 0,04 km³. Najveća mu je dubina 11 m, a izobata od 10 m se nalazi vrlo blizu ruba kopna. Oba su akvatorija povezana tjesnacem Karinsko tisno. Tjesnac je širok 110 – 250 m, dug je 2,5 km, dubina mu iznosi 12 m. S druge strane Novigradsko more spojeno je s Novskim ždrilom, kanjonom strmih obala, koje se obrušavaju ispod morske površine do dubine od 40 m. Dno je oba tjesnaca (Novsko ždrilo i Karinsko tisno) ravno i pjeskovito, uslijed snažnog protoka morske vode. Dno Novigradskog i Karinskog mora je pokriveno muljem. Značajna je činjenica da mulj s ušća Karinšćice ima ljekovita svojstva.
Područje je pod snažnim utjecajem kraških površinskih i podzemih voda, a duž obala izviru brojne vrulje. Raspon je godišnjeg protoka Zrmanje od 65 do 125 m³. Posebitost zemljopisnog položaja, prirodna zatvorenost i izoliranost te znatan priliv slatkovodnih dotoka rijekama Zrmanjom i Karinšćicom, potokom Slapača, vrelom Nozreti, jarugom Baščica i brojnim vruljama, utječe na bitno nižu slanost i temperaturu ovoga područja u svim sezonama osim ljeti.

Mudworm (Polydora websteri) parasitism is known to result in unsightly mud blisters in eastern oysters (Crassostrea virginica), resulting in reduced product quality and increased vulnerability to illness and environmental stress. While typically a concern only for bottom-grown oysters, an abnormal severe outbreak of P. websteri in surface-cultured oysters in New Brunswick, Canada, was reported in 2013, along with an anecdotally reported concurrent increase in siltation. Although heavier loads of silt are reported to increase P. websteri infestations in oysters, studies report mixed effects of siltation to this regard. Here, we report the results of a field experiment testing the effect of siltation on P. websteri recruitment in surface-grown oysters at an aquaculture site in Richibucto, New Brunswick. Live oysters of similar size were deployed at the aquaculture site and were left to collect P. websteri recruits under one of two siltation treatments (high vs. low) for 50 days. Results suggested that P. websteri recruitment correlated with met-rics of oyster size (shell weight, length, width, depth and surface area). When P. websteri counts were standardized for oyster shell morphometry, P. websteri recruitment was significantly higher in oysters exposed to the high siltation treatment, accumulating approximately 1.59 as many P. websteri as oysters exposed to the low siltation treatment. Our results suggest that higher amounts of siltation on surface-cultured oysters can result in increased rates of P. websteri parasitism. Enhanced cleaning regimes may help to alleviate the impacts of P. websteri in surface-grown oysters, although other mitigation strategies exist.

In UK estuaries conflicts have routinely occurred between economic and conservation interests regarding shellfish such as cockles Cerastoderma edule and mussels Mytilus edulis. The harvest of these species is economically important, but shellfish also constitute the main overwinter food supply of the oystercatcher Haematopus ostralegus. In this report we use a simplified spreadsheet model to predict the overwinter food requirements of oystercatchers in the Dee Estuary and compare the predictions of this model with those of an individual-based model which has been used to advise the setting of Total Allowable Catch in the Dee Estuary over recent years.
The models are based on the energy requirements of the birds and the energy value of their shellfish food. The spreadsheet model predicts the amount of shellfish required to maintain high survival rates within the oystercatcher population. The individual-based model predicts how the survival rate within the oystercatcher population is related to the amount of shellfish food and the amount removed by shellfishing. Although more complicated, the individual-based model represents the system in a more realistic way and can simulate specific shellfishing scenarios.
The models produced relatively similar predictions, especially when it was assumed that birds fed on upshore and terrestrial food in addition to cockles. As the biomass of cockles has declined since 2008, the models predicted that the amount required by the birds became close to the total available in 2012. The cockle biomass during 2013 was lower than that during 2012 and the spreadsheet model predicted that the birds required virtually all of the cockle stocks available.

The purpose of this project was to assess the mussel (Mytilus edulis) food requirements of oystercatcher (Haematopus ostralegus) in the Exe Estuary, which has been designated a Special Protection Area for overwintering waterbirds, including oystercatcher. The overwintering oystercatcher population of the Exe Estuary has been well-studied, and the birds are known to feed predominantly upon mussels in intertidal areas. There have been recent declines in the population size of oystercatcher in the Exe Estuary, mirroring wider declines throughout Europe, the reasons for which are unknown.

The study comprised:
• The collection of new data on the area of mussel beds, the density and size distribution of mussels on these beds, and the numbers and behaviour of oystercatcher on these beds;
• The collation of existing data on the food supply of oystercatchers in the Exe Estuary;
• The development of models to predict the food requirements of oystercatcher;
• Running simulations of the models to predict whether there is / could be any effect on oystercatcher survival of the current / potential future ways of managing the mussel fishery on the Exe Estuary.

The current mussel fishery on the Exe provides a feeding resource for oystercatcher on intertidal lays that are exposed on spring tides. Two potential management options that could be effective at improving the feeding conditions of oystercatcher would be to increase the number and area of intertidal mussel lays, and / or to place mussel discards at a relatively high shore level close to the oystercatcher roost.

This project documented a number of changes that have occurred to the Exe Estuary mussel and oystercatcher populations including:
• The number and size of mussel beds have decreased since traditional methods of maintaining mussel beds in the estuary have ceased.
• The density of mussels within the size range consumed by the birds has generally decreased, but the density of the larger mussels within this size range, which are more profitable to oystercatcher, has generally increased.
• Oystercatcher lose a higher proportion of mussels to attacks by carrion crows and herring gulls than they have in the past.
• The number of oystercatcher wintering in the estuary has declined, but the number of birds feeding on the mussel beds has been relatively stable.

The models developed in the project predict that the present day mussel population is sufficient to support the number of oystercatcher that were observed to feed on mussels.

The presence of mussel lays provides extra food for oystercatcher when these lays are exposed on spring tides. The present area, or increases in the area of mussel lays could increase the survival rate of oystercatcher if the number of birds feeding on mussels was over 2000. Below this threshold, starvation was predicted to affect 0 % of the population and so additional food resources cannot further reduce the starvation. The effect over 2000 birds is relatively small because the lays are only exposed for a short time, and so oystercatcher will obtain the majority of their food from mussel beds that are higher on the shore, and hence exposed for longer. Simulations were not run in which lays were positioned higher on the shore because this would not be commercially viable from a fishery perspective; the growth rate of mussels declines as they are positioned further up the shore because they are inundated with water for less time and so have less time to feed.

Factors that would affect the beneficial effect of discards include the size of the discards, the size of the discard bed and the date from which discards are replenished. Our simulations predicted that larger discards spread at lower density over a larger bed increased oystercatcher survival by the greatest amount. This happened because interference competition excluded some birds from smaller patches, and oystercatcher can maintain high intake rate down to low mussel densities. It is unlikely that the size of discards could be increased, but the simulations suggest that the greatest benefit to oystercatchers could be achieved by spreading discards over a larger area. Our simulations predicted that making discards available from January increased oystercatcher survival by the same amount as making them available from September. This was because the feeding conditions of birds deteriorate through winter as, for example, the ash-free dry mass of prey declines, interference competition intensifies and day length shortens. The intake rate of birds feeding on discards was not measured during the study, but we recommend that this is done to between understand the potential benefit of discards. We recommend that the best place for the discard bed would be along the top of the shore on an area of gravel (and hence of relatively low food value to the birds), to the south of Cockwood. This is south of an area where discards have been laid and exploited by oystercatcher in the past, but would experience lower levels of disturbance from human activity.

The biometrics, gut contents and sex ratio of the West African Mud Creeper, Tympanotonus fuscatus var radula from Mangrove swamps of
the University of Lagos Lagoon Front were studied for 12 months.
Tympanotonus fuscatus var radula has an elongated shell with regular
increasing whorls, weakly curved ribs and much fine striation with
blackish brown stripes on the shell. A total of 1135 samples were collected with highest species occurrence in August (11%). The total length ranged from 1.9 to 5.3 cm and total weight from 1.5 to 7.4g. The gastropod exhibited negative allometric growth with regression coefficient “b” of 1.19 values (b < 3). There was strong significant correlation between whorl number and shell length (Correlation
oefficient, r = 0.625, P < 0.01). Monthly condition factor (K) for the combined sex ranged between 6.36 and 7.85 with a mean of 7.07. Out of a total number of 1135 samples collected, 544 (48%) had empty stomachs. The index of relative importance (IRI) showed diatom, plant materials and organic debris as primary food items while bacteria and seaweed were secondary items. Hence, the species can be said to be a deposit feeder. Sex ratio obtained during the study was 1:2.3 which was significantly different from the expected 1: 1 ratio. Availability of more femalespermits continuous laying of eggs, thus making the species
available all year round.

In Prince Edward Island, the spring oyster fishery occurs from early May to mid-July. During this time, oysters may be harvested and held for multiple days by harvesters and are subsequently brought to oyster processors, who then relay them in seawater at leased sites. Over the past decade, however, processors have reported high mortalities of relayed catch during July. We investigated duration of air exposure as a potential driver for the high July mortalities reported by oyster processors.  Wild oysters were removed from fishing beds in June and July, 2017 and handled to mimic techniques used by harvesters. After removal, the oysters were exposed to atmospheric conditions (air) for 0, 24, 48, 72, or 96 hours and mortality was quantified. The oysters were then placed at a holding site for 60 days and mortality was quantified on days 7, 14, 30 and 60, and condition index was measured on remaining oysters from each air exposure treatment. Increased mortality was associated with increased air exposure duration, and this effect was most pronounced during July. Oysters exposed to air for 72-96 h in June experienced mortality rates of 4–11%, while the same air exposure duration in July yielded mortality rates four times higher than in June (16–44%). Most of the mortality associated with air exposure occurred during air exposure and within 7 days of placing oysters at the holding site. Condition index was also lower in July; however there was no evidence that oysters had spawned. Higher mortalities and lower condition indices in July were associated with higher average air and seawater temperatures (both ~3ºC higher than June). These results ultimately suggest that increased air exposure can result in substantial mortality of PEI oysters fished in July, and corroborate the reports of high July mortalities from oyster processors.

Kleptoparasitism involves the theft of resources such as food items from one individual by another. Such food-stealing behaviour can have important consequences for birds, in terms of individual fitness and population sizes. In order to understand avian host-kleptoparasite interactions, studies are needed which identify the factors which modulate the risk of kleptoparasitism. In temperate European intertidal areas, Eurasian oystercatchers (Haematopus ostralegus) feed primarily on bivalve molluscs, which may be stolen by kleptoparasitic species such as carrion crows (Corvus corone) and herring gulls (Larus argentatus). In this study we combined overwinter foraging observations of oystercatchers and their kleptoparasites on the Exe Estuary, UK, with statistical modelling to identify the factors that influence the likelihood of successful food stealing behaviour occurring. Across the winter, 16.4 % of oystercatcher foraging attempts ended in successful kleptoparasitism; the risk of theft was lowest in February (10.8 %) and highest in December (36.3 %). Using an information theoretic approach to compare multiple logistic regression models we present evidence that the outcome of host foraging attempts varied with the number of kleptoparasites per host within the foraging patch for two out of five individual months, and for all months grouped. Successful, kleptoparasitism was more likely to occur when the total number of all kleptoparasites per host was greater. Across the entire winter study period, oystercatcher foraging attempts that resulted in kleptoparasitism were associated with a mean number of kleptoparasites per host that was more than double that for foraging attempts that ended in the oystercatcher successfully consuming the mussel. Conversely, the stage of the tidal cycle within the estuary did not affect the outcome of oystercatcher foraging attempts. Our study provides evidence that bird numbers influence the risk of kleptoparasitism within avian assemblages.