Small Estuarine and Non-Estuarine Mangrove Ecosystems of Tanzania: Overlooked Coastal Habitats?

This research article published by Springer Nature Switzerland AG., 2016Small estuaries and non-estuarine habitats harboring mangroves are very important ecosystems which provide important ecosystem goods and services; such as provision of ecological niches for juvenile fishes and invertebrates, enhances fisheries, and in biodiversity conservation. Similar to large estuaries, they are highly perturbed which threatens their existence. This chapter uses beach seine, underwater visual census, and stable isotope data to discuss the importance of and threats to small estuaries and non-estuarine mangroves found in Dar es Salaam, Bagamoyo and Zanzibar, Tanzania. For example, mangroves of Kunduchi (Dar es Salaam) and Mbegani (Bagamoyo) which harbour predominantly higher densities of juveniles (≤10 cm) of two economically important species—Lutjanus fulviflamma and Lethrinus harak—than adjacent coral reefs. Evidence suggests further that the Kunduchi mangroves replenish fish populations on adjacent coral reefs; where over 90% and 29% of adult L. fulviflamma and L. harak individuals, respectively, have been identified to have lived in the mangroves as juveniles. In terms of habitat utilization by different size classes of fish, five of the 13 species (Lethrinus lentjan, L. variegatus, Pelates quadrilineatus, Siganus sutor and Sphyraena barracuda) found in Chwaka Bay (Zanzibar) were found as small-sized individuals in shallow and turbid mangrove areas with large juveniles and sub-adults in adjacent seagrass beds. The non-estuarine mangroves of Kunduchi and those of Mtoni estuary (Dar es Salaam) are subjected to pollution from urban activities. For example, stable isotope data of fishes indicate elevated levels of nitrogen in these mangroves with highest levels (δ15N = 15.2 ± 0.2) recorded in Mtoni estuary. In view of their importance and threats they face, these ecosystems require attention similar to large estuaries. If the current degradation rate of these ‘overlooked’ but equally important ecosystems continues, they may be declared ‘functionally disappeared’ in a few decades

Mangrove habitat use by juvenile reef fish: meta-analysis reveals that tidal regime matters more than biogeographic region

Published: December 31, 2014Identification of critical life-stage habitats is key to successful conservation efforts. Juveniles of some species show great flexibility in habitat use while other species rely heavily on a restricted number of juvenile habitats for protection and food. Considering the rapid degradation of coastal marine habitats worldwide, it is important to evaluate which species are more susceptible to loss of juvenile nursery habitats and how this differs across large biogeographic regions. Here we used a meta-analysis approach to investigate habitat use by juvenile reef fish species in tropical coastal ecosystems across the globe. Densities of juvenile fish species were compared among mangrove, seagrass and coral reef habitats. In the Caribbean, the majority of species showed significantly higher juvenile densities in mangroves as compared to seagrass beds and coral reefs, while for the Indo-Pacific region seagrass beds harbored the highest overall densities. Further analysis indicated that differences in tidal amplitude, irrespective of biogeographic region, appeared to be the major driver for this phenomenon. In addition, juvenile reef fish use of mangroves increased with increasing water salinity. In the Caribbean, species of specific families (e.g. Lutjanidae, Haemulidae) showed a higher reliance on mangroves or seagrass beds as juvenile habitats than other species, whereas in the Indo-Pacific family-specific trends of juvenile habitat utilization were less apparent. The findings of this study highlight the importance of incorporating region-specific tidal inundation regimes into marine spatial conservation planning and ecosystem based management. Furthermore, the significant role of water salinity and tidal access as drivers of mangrove fish habitat use implies that changes in seawater level and rainfall due to climate change may have important effects on how juvenile reef fish use nearshore seascapes in the future.Mathias M. Igulu, Ivan Nagelkerken, Martijn Dorenbosch, Monique G. G. Grol, Alastair R. Harborne, Ismael A. Kimirei, Peter J. Mumby, Andrew D. Olds, Yunus D. Mgay

New morphological information on, and species of placoderm fish Africanaspis (Arthrodira, Placodermi) from the Late Devonian of South Africa

Here we present a new species of placoderm fish, Africanaspis edmountaini sp. nov., and redescribe Africanaspis doryssa on the basis of new material collected from the type locality of Africanaspis. The new material includes the first head shields of Africanaspis doryssa in addition to soft anatomy for both taxa. Hitherto Africanaspis was entirely described from trunk armour and no record of body and fin outlines had been recorded. In addition the first record of embryonic and juvenile specimens of Africanaspis doryssa is presented and provides a growth series from presumed hatchlings to presumed adults. The presence of a greater number of juveniles compared to adults indicates that the Waterloo Farm fossil site in South Africa represents the first nursery site of arthrodire placoderms known from a cold water environment. The preservation of an ontogenetic series demonstrates that variation within the earlier known sample, initially considered to have resulted from ontogenetic change, instead indicates the presence of a second, less common species Africanaspis edmountaini sp. nov. There is some faunal overlap between the Waterloo Farm fossil site and faunas described from Strud in Belgium and Red Hill, Pennsylvania, in north America, supporting the concept of a more cosmopolitan vertebrate fauna in the Famennian than earlier in the Devonian

Orientation from open water to settlement habitats by coral reef fish: Behavioral flexibility in the use of multiple reliable cues

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The potential role of visual cues for microhabitat selection during the early life phase of a coral reef fish (Lutjanus fulviflamma)

Pelagic larvae of various coral reef fish species are known to be active swimmers that can carefully select preferred microhabitat during settlement. Fish larvae may use visual, environmental, or chemical cues to orient and settle in shallow-water habitats such as coral reefs, mangroves, and seagrass beds. In the present study, we investigated in dual-choice laboratory experiments the visual attraction of recent settlers of a reef fish (Lutjanus fulviflamma) towards different microhabitats, conspecifics, and heterospecifics, and the interactive effects among these cues. Fish preferred seagrass and coral above mangrove roots. Fish were more attracted towards a combination of conspecifics or heterospecifics and seagrass microhabitats than to seagrass microhabitats alone, but showed a significantly stronger preference for conspecifics than for heterospecifics when placed in preferred seagrass or non-preferred mangrove microhabitats. However, the preference for conspecifics disappeared when choice was given between conspecifics placed in non-preferred mangrove microhabitat versus heterospecifics placed in preferred seagrass micro habitat. A multiple choice experiment further showed that recent settlers preferred conspecifics of equal or about 1. cm larger body size, but not of 2 or 3. cm larger body size. The higher attraction towards the combination of seagrass microhabitats and conspecifics/heterospecifics shows an additive effect of the latter, which could be explained by the fact that presence of resident fish in preferred habitat may indicate favorable conditions in the field and may offer an increased safety through schooling. However, (1) attraction towards conspecifics of (nearly) similar body size and not larger, (2) stronger attraction towards conspecifics in a non-preferred than in a preferred microhabitat, and (3) equal attraction towards conspecifics in non-preferred microhabitat vs. heterospecifics in preferred microhabitat all indicate that the importance of schooling with conspecifics is highly context-dependent. This may have significant effects on the habitat selection and distribution of early-stage fish (and thus also on consecutive life stages) in coastal habitats. Our findings point to the potential ecological significance of various visual cues, and their interactive effects, for early juvenile coral reef fishes while settling into shallow-water habitats and/or selecting early post-settlement habitats. © 2011 Elsevier B.V.M.M. Igulu, I. Nagelkerken, R. Fraaije, R. van Hintum, H. Ligtenberg, Y.D. Mgay

Preliminary findings on the food and feeding dynamics of the anchovy Stolephorus commersonnii (Lacepède, 1803) and the Indian mackerel Rastrelliger kanagurta (Cuvier, 1817) from Tanga Region, Tanzania

Small pelagic fishes play an important role in the ecosystem by linking planktonic production and higher trophic level predators, and provide a livelihood to both the small-scale and commercial fisher communities. This study analyzed the food and feeding habits of Stolephorus commersonnii (Lacepède, 1803) and Rastrelliger kanagurta (Cuvier, 1817) from the ring-net fishery in Tanga, Tanzania. A total of 1 434 and 320 stomachs of S. commersonnii and R. kanagurta respectively were examined for gut contents using the relative volumetric method. S. commersonnii was found to be a planktivorous carnivore, feeding principally on planktonic penaeid shrimps (48.6%), fish larvae (33.2%) and zooplankton (12.3%). R. kanagurta was found to be carnivorous, feeding predominantly on fish (60.6%), mainly S. commersonnii, while penaeid shrimps, juvenile fish, and juvenile stages of squids formed 26.5% of the total number of food items in R. kanagurta guts. Both S. commersonnii and R. kanagurta exhibited ontogenic diet shifts, where they fed exclusively on small prey as juveniles and consumed larger food items as they grew. The index of vacuity was higher in S. commersonnii (46.1%) than in R. kanagurta (16.6%). This study revealed the importance of penaeid shrimps as food for S. commersonnii, that in turn formed the main food for R. kanagurta. This implied that the two species were able to coexist in the same niche by avoiding interspecific competition for food

Reproductive biology of the anchovy (Stolephorus commersonnii, Lacepède, 1803) and spotted sardine (Amblygaster sirm, Walbaum, 1792) from Tanga Region, Tanzania

The present study investigated the reproductive biology of Stolephorus commersonnii and Amblygaster sirm at two landing sites in Tanga on the northern coast of Tanzania. Fish samples were collected on a monthly basis from ringnets operated by artisanal fishers in the nearby coastal waters. Spawning seasons were determined using gonadosomatic index (GSI) and gonadal maturity stages. The size at first maturity was 57.7 mm and 66.2 mm total length for male and female S. commersonnii respectively. Male and female A. sirm were estimated to attain first maturity at 147.7 mm and 169.2 mm respectively. The spawning seasons of both species were protracted. S. commersonnii demonstrated a year round spawning cycle with peaks in August, October and January. The peak spawning season for male and female A. sirm was recorded in August and September respectively. Both species exhibited skewed size-dependent sex ratios with females predominating in the larger size classes. A. sirm had a higher fecundity rate with a maximum of 96,500 eggs in the largest female fish of 258 mm as compared to S. commersonnii (10,055 eggs) in the largest fish of 98 mm. The mean (±SE) total fecundity of S. comersonnii and A. sirm was 5,134.7 ± 136.9 eggs, and 47,029.03 ± 1,435.13 eggs in females of sizes 68 mm to 98 mm and 170 mm to 258 mm respectively

Mean effect size ± SE (based on variability across species) for densities of juvenile nursery species in different habitats as a function of tidal amplitude for the Caribbean (filled squares) and Indo-Pacific (open squares) regions, for a) seagrass (SG) – mangrove (MG) comparison, b) coral reef (RF) – seagrass comparison, and c) coral reef – mangrove comparison.

<p>Numbers in the graphs indicate: Belize (1), Curaçao (2), Aruba (3), Grand Cayman (4), Florida (5), Turks and Caicos Islands (6), Bimini (7), San Salvador (8), Andros (9), Abaco (10), Bermuda (11), Lee Stocking Island (12), Solomon Islands (13), Wakatobi (14), Ryukyu Islands (15), Moreton (16), Palm Islands (17), Kunduchi (18), Mafia (19), Mbegani (20) Zanzibar (21) and Pemba (22).</p

Cluster analysis plot based on Euclidean distances using Hedges' <i>d</i> values for all three habitat comparisons per fish species.

<p>Species from the Caribbean and Indo-Pacific are combined. Species for which one or more habitat comparisons were absent were omitted as no distance measure could be calculated.</p

Summary statistics of a random-effects model for overall comparison of juvenile (1–10 cm, TL) fish densities in seagrass beds (SG), mangroves (MG), and coral reefs (RF).

<p>Values indicate weighted mean effect size (<i>d</i>₊), 95% confidence intervals (CI), bias-corrected 95% confidence intervals (Bias CI), total heterogeneity (<i>Q_T</i>), and degrees of freedom (<i>df</i>). The publication bias test is based on Spearman rank-order correlation (Rs); R =  Rosenthal's methods for calculating fail-safe numbers. Significant p-values indicate significant heterogeneity within a mean effect size; * indicates that the mean effect size is significantly different from zero.</p><p>Summary statistics of a random-effects model for overall comparison of juvenile (1–10 cm, TL) fish densities in seagrass beds (SG), mangroves (MG), and coral reefs (RF).</p