Essential spawning grounds of Scottish herring: current knowledge and future challenges

Scotland once had the largest herring fishery globally, generating local income, identity, and societal change. Following historic stock collapse, in spring 2018/2019 large herring shoals were observed on the west coast for the first time in decades, at a formerly important spawning ground. This highlights the urgency of maintaining historic (and contemporary) benthic spawning habitat, which these fish rely upon, in good condition. However, information on exact location, characteristics, and status of historic and contemporary spawning grounds, if existing, is not easily accessible. We searched over 1190 literature sources, dating back to 1884, using scientific databases and web-based searches, and ran a query for automated search of comprehensive historic reports. We present current knowledge on Scottish herring spawning grounds, retrieved through these searches and fisher interviews, maps showing historic and contemporary spawning grounds, and discuss challenges arising from the methods used to recognize these grounds. Knowledge gaps regarding location and environmental status of past and current spawning grounds, particularly relevant for Scotland’s west coast, are identified. Based on the importance of specific environmental and physical variables for herring reproductive success, we advocate the inclusion of essential spawning grounds into herring management plans. This will require additional data on spawning grounds, including historic local ecological knowledge rarely considered. An inclusive ecosystem-based approach to herring management would allow more targeted actions to conserve (and potentially restore) essential spawning habitat. More effective management strategies will also call for reversing the (global) issue of the disconnect between different stakeholder groups

Growth and mortality of the exploited mangrove crab Ucides cordatus (Ucididae) in N-Brazil.

Reliable estimates of growth and mortality parameters are crucial for understanding how populations of exploited species may respond to shifts in fishing pressure. Results of previous studies on growth of the ecologically and economically important mangrove crab Ucides cordatus in Brazil differ strongly and age estimates for minimum legal capture size (60 mm carapace width) range between less than 1 and 6–10 years. These discrepancies are probably due to inherent problems of the applied methods, namely laboratory based observations and cohort-analyses. The present paper takes a new approach by measuring individual growth increments of U. cordatus in situ. Crabs were measured, tagged, released into 100 m² field enclosures and periodically recaptured. There was no indication of an enclosure effect and 209 growth increments were obtained from specimens measuring 20.5 to 89.5 mm in carapace width (CW). For improving the size coverage of the von Bertalanffy growth curve, first instar juveniles were reared in the laboratory up to an age of six months. Their average percent size increase (PI) was 22.57 ± 6.75% during biweekly measurements, resulting in a CW of 7.8 to 11.5 mm after six months. Average PI of the enclosure crabs ranged between 10.87 ± 1.17% (indiv. 80 mm CW) per moult and two large females had moulted without growing. Growth analysis revealed a larger asymptotic size in males than in females (89 mm versus 72 mm) while the growth parameter K of the von Bertalanffy growth function (VBGF) was lower in males (0.17 versus 0.25), resulting in similar growth performance of the two sexes (males: 1.16; females: 1.10). Age at legal minimum capture size estimated by the inversed VBGF was 6.13 years in males and 7.38 years in females, corroborating the mid to upper range of earlier growth estimates. Mortality parameters (Z, M and F) were calculated using length converted catch curves. Z in males was higher than in females (0.69 versus 0.49), as expected from the male-biased fishery. M, F and the exploitation rate E were estimated for three different scenarios taking into account the uncertainty of the age/size of functional maturity. Our study provides key information for the sustainable management of the U. cordatus fishery and confirms that the species is relatively slow growing and long-lived (> 10 years), suggesting a high vulnerability to overfishing

Intraspecific facilitation of the recruitment of a burrowing mangrove crab species along an environmental gradient

The exploited mangrove crab Ucides cordatus, an important ecosystem engineer in South American mangroves, has a biphasic lifecycle with pelagic larvae developing offshore. Megalopae return to the mangrove forest to settle, i.e., metamorphose into benthic juveniles but its environmental drivers remain poorly understood. We postulate that conspecific crabs facilitate recruitment. In the field, we investigated whether the number of recruits is higher near conspecific burrows than in areas without conspecific bioturbation. Recruit sampling was conducted monthly from April 2008 to May 2009 along an environmental gradient. First, U. cordatus recruits of that year emerged from March to July. Intraspecific facilitation was indicated by significantly higher recruit numbers in bioturbated than in non-bioturbated substrates. Recruits were most abundant in zones with intermediate conditions of inundation, leaf litter standing stock, sediment consistency, luminosity, temperature, salinity, and pH. Avoidance of more inundated zones likely reduces predation by fishes and of less inundated zones helps individuals escape more stressful, drier environmental conditions. The observed habitat-specific recruitment pattern must be considered when designing field assessments of the population structure of U. cordatus (which should include sampling recruits) and enhancing stocks by releasing laboratory-cultivated larvae and first juveniles into the wild

Effects of geophysical cycles on the rhythm of mass mate searching of a harvested mangrove crab

The harvested mangrove crab Ucides cordatus exhibits conspicuous cycles of searching for mates en masse. This phenomenon, called andada, always occurs at the syzygies, but the particular moon phase, full, new or rarely both, varies for unknown reasons. The presence and absence of andada activities of a Northeast Brazilian crab population was assessed by boat between 2006 and 2011. In 2008 crabs were additionally collected in sampling plots to determine the duration and intensity of andada events and in 2010 the timing of andada with respect to the light:dark cycle and tidal cycle was studied by observations from platforms. We found that andada occurred during the day and night and that the rhythm of mate searching was linked to the syzygy tide inequality cycle (STIC). Andada shifted between new and full moon, depending upon which moon phase had the higher amplitude tides. The ultimate cause of andada is likely to be increased larval survival after synchronous release at highest amplitude spring tides 1 month later. Such anticipatory behaviour is probably under endogenous control. The results of this study can help to improve temporal placements of capture bans for this harvested species and reduce current conflicts between fishers and regulatory agencies

First field record of mangrove crab Ucides cordatus (Crustacea: Decapoda: Ucididae) recruits co-inhabiting burrows of conspecific crabs

Recruits of the mangrove crab Ucides cordatus (Linnaeus, 1763), rarely encountered in the field were found co-inhabiting burrows of larger male and female conspecifics in the mangrove forest. They were located in the sediment of the inner walls and burrow plugs. Average carapace width (CW) of the hosting and co-inhabiting crabs was 3.8 ± 0.20 and 0.9 ± 0.03, respectively. As shown by the size-frequency distribution, while most recruits leave the conspecific burrows after reaching 1.0 cm CW, some stay until they reach a size of 2.5 cm CW. The results of this study contribute to a better understanding of recruitment patterns in this ecologically and economically important mangrove crab species. Follow-up studies are however needed to fully determine the role of conspecific burrows for juvenile habitat choice and survivorship in U. cordatus

Artificial Crab Burrows Facilitate Desalting of Rooted Mangrove Sediment in a Microcosm Study.

Water uptake by mangrove trees can result in salt accumulation in sedimentaround roots, negatively influencing growth. Tidal pumping facilitates salt release and canbe enhanced by crab burrows. Similarly, flushing of burrows by incoming tidal waterdecreases sediment salinity. In contrast to burrows with multiple entrances, the role of burrowswith one opening for salinity reduction is largely unknown. In a microcosm experiment westudied the effect of artificial, burrow-like macro-pores with one opening on the desalting ofmangrove sediment and growth of Rhizophora mangle L. seedlings. Sediment salinity,seedling leaf area and seedling growth were monitored over six months. Artificial burrowsfacilitated salt release from the sediment after six weeks, but seedling growth was notinfluenced. To test whether crab burrows with one opening facilitate salt release in mangroveforests, sediment salinities were measured in areas with and without R. mangle stilt roots inNorth Brazil at the beginning and end of the wet season. In addition, burrows of Ucidescordatus were counted. High crab burrow densities and sediment salinities were associated with stilt root occurrence. Precipitation and salt accumulation by tree roots seem to have alarger effect on sediment salinity than desalting by U. cordatus burrows

O efeito da salinidade no desenvolvimento larval do caranguejo - uçá, Ucides cordatus (Linnaeus, 1763) (Decapoda: Ocypodidae) no Norte do Brasil

The present work studied the effect of salinity on the survival and duration of larval development of the mangrove crab, Ucides cordatus (from the Caeté River estuary, North of Brazil) until the megalopal phase in seven salinity treatments (0, 5, 10, 15, 20, 25 e 30). Salinity significantly affected the survival of the zoea larvae, however it did not affect the duration of the larval development (20.77 ± 1.56 days). In salinity 0, 5 and 10 all zoea larvae died. Only from off salinity 15, complete development until the megalopal phase occurred. The survival rate was highest in salinity 30 (72%) and lowest in 15 (16%). The reduced survival rate of the U. cordatus zoea larvae, in low salinities, indicates the necessity of larval dispersion from the estuary to coastal waters, where salinity conditions for larval development are more favorable. Otherwise, if there was no dispersion, the reduced salinity of estuarine waters in the rainy season would cause a high mortality thereby affecting the recruitment, maintenance and growth of the U. cordatus population in the mangroves

Effects of substrata and conspecific odour on the metamorphosis of mangrove crab megalopae, Ucides cordatus (Ocypodidae)

Environmental cues associated with the adult habitat induce metamorphosis in many marine invertebrate larvae and thus settlement in a place appropriate for completing their life cycle. Ucides cordatus is a semi-terrestrial crab that exemplifies organisms living in mangrove estuaries as adults, but whose larvae develop offshore and return to the estuarine environment prior to metamorphosis. We investigated the influence of substrata (burrow mud, channel-bank mud, dead wood, sand, nylon mesh) and conspecific adult cues (crab-conditioned seawater) on metamorphosis and the duration of the megalopal phase. Our results suggest that U. cordatus megalopae settle in areas populated by conspecific crabs and/or muddy habitats. High moulting rates were observed in the treatments with mud and/or adult odour (67–91%), whereas in the other substrata treatments and in the seawater control less than 20% and 5% of the megalopae metamorphosed, respectively. Megalopae are also capable of delaying metamorphosis. Over channel-bank mud they moulted 8 days later at an average, compared to the treatment with crab-conditioned seawater. Such a delay could result in a reduced juvenile fitness. In seawater control almost all megalopae died prior to metamorphosis, without moulting spontaneously as observed in other species. This reflects the crucial importance of habitat cues for the settlement and recruitment of this ecologically and economically important species. We outline the necessity of future research concerning post-metamorphic carry-over effects, the possible role of biofilms in absorbing and/or producing settlement cues, as well as the question whether the megalopae react specifically to the odour of U. cordatus, or also to the one of other crab other species. The latter is of particular interest against the background of the spreading lethargic crab disease that results in massive mortalities of U. cordatus, but not of co-occurring-species

Salinity tolerance of northern Brazilian mangrove crab larvae, Ucides cordatus (Ocypodidae): Necessity for larval export?

The life cycle of the semiterrestrial mangrove crab Ucides cordatus includes pelagic larvae that are released into estuarine waters during the wet season and who may thus encounter potentially stressful low and variable salinity conditions. The effect of salinity on the survival of the zoea larvae, the number of zoeal stages and the duration of development from hatching to megalopa was experimentally studied by rearing larvae from the Caeté estuary, Northern Brazil, in seven salinity treatments (0, 5, 10, 15, 20, 25, and 30). For a better interpretation of the laboratory results, estuarine salinities were measured over five consecutive years during the species' reproductive season. The survival of the zoea larvae varied significantly with salinity, while the number of stages and the duration of their development remained constant. Development to megalopa took 20.77 ± 1.57 days and comprised five zoeal stages with ZI and ZII being euryhaline and later stages stenohaline. The newly hatched larvae stayed alive for up to 6 days in freshwater (average 4.32 ± 0.82 days), but did not moult to the second zoeal stage. ZII larvae first occurred from salinity 5 onwards and later zoeal stages at all tested salinities ≥10. However, the larvae only survived to megalopa at salinities ≥15, with highest numbers at salinity 30 (72%) and lowest at 15 (16%). Lethal salinities ≤10 occurred frequently in the estuary during the reproductive season. This suggests a need for larval export to offshore and thus more saline waters to allow for significant larval survival and maintenance of viable populations of this commercially important species. A regional rather than local approach for management is suggested due to the likelihood of long distance larval dispersal by offshore currents