Fish Culture Zone Research Articles

A remotely controlled automated field measurement system for light extinction in coastal waters

The Secchi disk depth (SD) is an important parameter in aquatic ecosystem monitoring. As algal growth depends on solar irradiation, the SD - a measure of light extinction - gives an indirect indication of the chlorophyll concentration. However, most SD measurements are manually based and too sparse to resolve water quality variations during algal blooms.A remotely controlled automatic system for field measurement of light extinction has been developed and installed in three marine fish culture zones in Hong Kong. The visual images of the disk at different prescribed depths and the surrounding water are taken. Based on the contrast theory and image analysis, the recorded light intensity distributions can be analyzed to give the SD and the light extinction coefficient. The method has been extensively verified by field data over a wide range of water quality and hydro-meteorological conditions. The proposed system enables high frequency SD measurements on demand for environmental management and emergency response.

Uncertainty evaluation and performance assessment of water quality model for mariculture management.

Ecosystem based water quality models are important tools for prognostic site assessment and evaluation of ecosystem-performance of marine fish farms. We present the development and application of a comprehensive Fish Culture Zone Water Quality Model using continuous bi-weekly field data over a six-year period (2012-2017). The model simulates five interacting subsystems: phytoplankton, phosphorus and nitrogen cycles, and the dissolved oxygen (DO) and particulate organic carbon balance. The application of the model to two fish culture zones in Hong Kong shows the model captures the trends of nutrient and DO variation and the performance in quantitative prediction of algal biomass is challenging. The effect of errors in the specification of primary model inputs are evaluated using dimensionless sensitivity coefficients based on First Order Variance Analysis reveals the relative importance of fish stock (loading), physical size (volume), tidal flushing rate and boundary conditions in the prediction of key water quality variables.

Real-time automated identification of algal bloom species for fisheries management in subtropical coastal waters

Harmful Algal Blooms (HAB) pose significant challenges to fisheries management and food and water security. The onset of a HAB is notoriously difficult to predict. Traditional methods of algal species identification under a microscope are also laborious and time consuming. A real-time system for identification and concentration measurement of algal bloom species has been developed at a marine fish culture zone (FCZ) in the subtropical coastal waters of Hong Kong. The system is based on analysis of high frequency algal cell images obtained from an underwater Imaging FlowCytobot (IFCB) deployed on the fish farm. An explainable supervised machine learning technique has been successfuly developed. The algal species classifier is trained by presenting a wide range of extracted image features to a random forest algorithm. An optimized set of 25 features is identified by a recursive feature elimination technique. The random forest (RF) classifier can identify 15 target HAB classes with an overall out-of-bag accuracy of 94.2%, with individual F1 score ranging from 0.8 to 1.0. The classifier performs equally well as a Convolution Neural Network (CNN) developed using transfer learning techniques. Based on the classifier, an automated real-time species identification and cell counting protocol has been developed, with a response time of 10 min after data collection. This work represents the first successful attempt of continuous algal species monitoring by IFCB and artificial intelligence (AI) – based detection of HAB in subtropical coastal waters.

Potential bioremediation effects of seaweed Gracilaria lemaneiformis on heavy metals in coastal sediment from a typical mariculture zone.

Seaweeds are good bio-monitors of heavy metals pollution in coastal seawater. In the present study, the potential bioremediation effects of cultivated Gracilaria lemaneiformis on heavy metals in Nan'ao coastal sediment from a typical mariculture zone, South China were evaluated. Sediment samples were collected from five different zones (Gracilaria cultivation zone, G; Fish culture zone, F; Shellfish culture zone, S; Transition zone, T; Control zone, C) from December 2014 to July 2015. The concentrations of Cd, Pb, Cu, and Zn in the sediments were significantly different among the various types of mariculture areas. The concentrations varied widely: Cd (0.04-1.02) μg g-1; Cu (1.19-37.70) μg g-1; Pb (8.45-74.45) μg g-1; Zn (36.80-201.24) μg g-1. The lowest heavy metal concentrations in the sediment were occurred at Gracilaria cultivation zone, while higher concentrations occurred at control zones and fish culture zones. The pollution load index, principal components and cluster analysis showed that heavy metal concentrations were the highest at fish culture zone, while the concentrations were the lowest at Gracilaria cultivation zone, and Gracilaria cultivation affects the heavy metals in the sediments. Gracilaria had strong adsorption capacities for heavy metals from seawater, showing the highest heavy metal Bioconcentration Factors in May (higher seaweed biomass period). Consequently, the results suggested that Gracilaria cultivation influences the heavy metal concentrations in sediments from the typical coastal mariculture zone. Gracilaria cultivation has the potential to bioremediate heavy metals in the coastal sediments. Therefore, Gracilaria cultivation can add environmental advantages and ecological values to coastal mariculture zones.

Effects of fish culture on particulate organic matter in a reservoir-type river as revealed by absorption spectroscopy and fluorescence EEM-PARAFAC

Dam construction and fish culture can change the biogeochemical processes in river, yet their impact on the spectral properties of particulate organic matter (POM) remains to be studied. This was investigated in a reservoir-type river (Minjiang river, SE China) using absorption spectroscopy and fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). Five fluorescent components were identified from POM with PARAFAC. Four components C1–C4 were affected by the seasonal variations of rainfall and runoff, indicating the influences of hydrological condition and terrestrial inputs. The Chlorophyll a concentration (Chl a) correlated significantly with the humic-like C3 (p < 0.05) and the protein-like C4 (p < 0.01), indicating phytoplankton was an important source of C3 and C4. The Chl a and fluorescence intensities of C3–C4 were higher in the fish culture zones than in other areas, and the absorption coefficient a300 and C1–C4 were lower downstream the dam. These results indicated that fish farming in the reservoir probably promoted the production of POM. The a300 and C1 per unit mass of suspended particulate matter (a300/TSM and C1/TSM) correlated significantly with the median particle size (p < 0.01), which might be related to the contribution of micro-phytoplankton. The absorption spectra of POM showed a shoulder peak at ∼280 nm, and its intensity correlated significantly and positively with Chl a (p < 0.01). These results indicated that the peak was probably derived from phytoplankton production. Our results have implications for better understanding the influences of human activities on the dynamics of river POM.

Nutrient release from fish cage aquaculture and mitigation strategies in Daya Bay, southern China

Finfish cage culture is the most predominant form of mariculture. The rapid expansion of fish cage culture systems has raised concerns about their environmental impact, such as nutrient release. In this study, for the first time, we estimated the release of nitrogen (N) and phosphorus (P) from fish cage culture in Daya Bay, southern China, by constructing N and P budget models based on a mass balance principle. In addition, the contribution and importance of nutrients from fish culture and other nutrient sources, including submarine groundwater discharge, benthic sediments, local rivers, and atmospheric deposition were assessed. The annual amount of N and P released from fish cage culture was 205.6 metric tons (hereafter tons) of N and 39.2 tons of P, including 142.7 tons of dissolved inorganic nitrogen (DIN) and 15.1 tons of dissolved inorganic phosphorus (DIP). Among the analyzed nutrient sources, the contributions of DIN and DIP from fish culture were 7.0% and 2.7%, respectively. For cages consuming conventional trash fish, 142 kg of N and 26 kg of P were released into the environment per ton of fish products, much higher than the values (72 kg N and 17.3 kg P) for cages using formulated feed. In fish culture, the dissolved nutrients were more N rich, but the particulate nutrients were more P rich. The ratio of cage-derived N and P was 21.1, higher than the ratio of coastal seawater (27.1), indicating that cage culture may also impact the local nutrient forms around farming regions. Oyster cultivation and harvest removed 126.3 tons of N and 35.1 tons of P from of the bay. Replacing trash fish with formulated feed and co-culturing of nutrient extractive species (e.g., bivalves, macroalgae) and deposit-feeding species (e.g., sea cucumber) in fish culture zones can be efficient nutrient mitigation strategies.

Metabolic and phylogenetic profiles of microbial communities from a mariculture base on the Chinese Guangdong coast

This study examined the phylogenetic and metabolic features of microbial communities in Baisha Bay mariculture areas of Nan’ao Island, a mariculture base on the Chinese Guangdong coast. Large seaweed (Gracilaria lemaneiformis) cultivation zones (LZ), fish culture zones (FZ), and control zones (CZ) were sampled. According to community-level physiological profiling, d-mannitol, d-mannitol/α-d-lactose, and d-mannitol/α-cyclodextrin were the carbon sources with the highest utilization rates in CZ, LZ, and FZ, respectively (Rs > 1.9). Denaturing gradient gel electrophoresis analysis showed that the dominant bacteria in CZ and LZ were Alphaproteobacteria (39.9 and 34.5%, respectively), whereas in FZ they were Flavobacteria (32.2%). The Shannon diversity in CZ and LZ was significantly higher than in FZ (P < 0.05). Some opportunistic pathogens, such as Vibrio, Pseudoalteromonas and Loktanella, were abundant in FZ. The microbial communities and most of the dominant bacteria in FZ were more affected by environmental factors including salinity, chemical oxygen demand and total phosphorus. Some bacteria, such as Brevundimonas and Polaribater, have the potential to degrade agar in seaweed, and were isolated from samples rich in algae which are dominant in LZ. The cultivation of G. lemaneiformis created the notable structures and functions of macroalgae-associated bacterial communities that were less affected by the environmental factors examined.

Comparative assessment of water quality parameters of mariculture for fish production in Hong Kong Waters

The objective of the study is to evaluate the effect of fish cultivation on water quality in fish culture zone (FCZ) and analysed by Principle Component Analysis (PCA). 120 surface water samples were collected from Hong Kong Waters (60 samples in Victoria Harbour and another 60 in Ma Wan FCZ). Significant difference was found in dissolved oxygen (MW: 59.6%; VH: 81.3%), and Escherichia coli (MW: 465CFU/100ml; VH: 162.5CFU/100ml). Three principle components are responsible for water quality variations in the studying sites. The first component included E. coli (0.625) and dissolved oxygen (0.701). The second included E. coli (0.387) and ammonical-nitrogen (0.571). The third included E. coli (0.194) and ammonical-nitrogen (0.287). This framework provides information to assess the relative contribution of eco-aquaculture to nutrient loads and the subsequent risk of eutrophication. To conclude, a rigorous monitoring of water quality is necessary to assess point and nonpoint source pollution. Besides, appropriate remediation techniques should be used to combat water pollution and achieve sustainability.

Tidal flushing time of marine fish culture zones in Hong Kong

Accurate determination of flushing time is crucial for maintaining sustainable production in fish culture zones (FCZs), as it represents the physical self-purification capability via tidal exchange with clean water in the outer sea. However, owing to the temporal and spatial complexity of the coastal flushing process, existing methods for determining flushing time may not be generally applicable. In this paper, a systematic method for determining the flushing time in FCZs is presented, in which bathymetry, runoff, tidal range and stratification are properly accounted for. We determine the flushing time via numerical tracer experiments, using robust 3D hydrodynamic and mass transport models. For FCZs located in sheltered and land-locked tidal inlets, the system boundary can be naturally defined at the connection with the open sea. For FCZs located in open waters, hydrodynamic tracking is first used to assess the extent of tidal excursion and thus delimit the initial boundary between clean water and polluted water. This general method is applied to all designated marine FCZs in Hong Kong for both the dry and wet seasons, including 20 sheltered FCZs (in semi-enclosed waters of Tolo Harbour, Mirs Bay, and Port Shelter) and 6 FCZs in open waters. Our results show that flushing time is the longest in inner Port Shelter (about 40 days in dry season), and the shortest for the FCZs in open waters (less than one week in dry season). In addition, the flushing time in dry season is commonly longer than that in wet season: 20%∼40% for most well-sheltered FCZs; 2.6∼4 times for the others. Our results indicate a positive correlation between the flushing time and distance to open boundary, supporting the view that the flushing time of a FCZ is closely related to its location. This study provides a solid basis for mariculture management such as the determination of carrying capacity of FCZs.

The extended Kalman filter for forecast of algal bloom dynamics

A deterministic ecosystem model is combined with an extended Kalman filter (EKF) to produce short term forecasts of algal bloom and dissolved oxygen dynamics in a marine fish culture zone (FCZ). The weakly flushed FCZ is modelled as a well-mixed system; the tidal exchange with the outer bay is lumped into a flushing rate that is numerically determined from a three-dimensional hydrodynamic model. The ecosystem model incorporates phytoplankton growth kinetics, nutrient uptake, photosynthetic production, nutrient sources from organic fish farm loads, and nutrient exchange with a sediment bed layer. High frequency field observations of chlorophyll, dissolved oxygen (DO) and hydro-meteorological parameters (sampling interval Δ t = 1 day, 2 h, 1 h, respectively) and bi-weekly nutrient data are assimilated into the model to produce the combined state estimate accounting for the uncertainties. In addition to the water quality state variables, the EKF incorporates dynamic estimation of algal growth rate and settling velocity. The effectiveness of the EKF data assimilation is studied for a wide range of sampling intervals and prediction lead-times. The chlorophyll and dissolved oxygen estimated by the EKF are compared with field data of seven algal bloom events observed at Lamma Island, Hong Kong. The results show that the EKF estimate well captures the nonlinear error evolution in time; the chlorophyll level can be satisfactorily predicted by the filtered model estimate with a mean absolute error of around 1–2 μg/L. Predictions with 1–2 day lead-time are highly correlated with the observations ( r = 0.7–0.9); the correlation stays at a high level for a lead-time of 3 days ( r = 0.6–0.7). Estimated algal growth and settling rates are in accord with field observations; the more frequent DO data can compensate for less frequent algal biomass measurements. The present study is the first time the EKF is successfully applied to forecast an entire algal bloom cycle, suggesting the possibility of using EKF for real time forecast of algal bloom dynamics.

Okadaic acid, a causative toxin of diarrhetic shellfish poisoning, in green-lipped mussels Perna viridis from Hong Kong fish culture zones: Method development and monitoring

Green-lipped mussels ( Perna viridis) were collected from seven fish culture zones (FCZs) in Hong Kong and analyzed for okadaic acid (OA). A conventional HPLC method was modified by incorporating a proteinase K digestion step. Results suggest that a higher recovery (2.5 times higher) of OA was obtained from spiked samples after the addition of 1.08 mg proteinase K in comparison with samples incubated without the proteolytic enzyme. For the hepatopancreas (HP) of individual field-collected mussels, the additional digestion step can enhance OA extraction by 3.1 times. Spatial and temporal variations in OA concentrations in the mussels from various FCZs were investigated. The highest concentration of OA in mussel HP samples was 1164.9 ng/g HP wet wt. With respect to OA concentrations in whole mussel tissues from seven sites and four seasonal samplings, the concentrations were between 70.0 and 131.0 ng/g wet wt., which did not exceed the generally recognized international regulatory criteria (>200 ng/g) for OA.

Effects of nutrient enrichment derived from fish farming activities on macroinvertebrate assemblages in a subtropical region of Hong Kong

To study the correlation between nutrient enrichment derived from fish farming activities and changes in macrobenthic assemblages, a one-year field study was conducted in Kau Sai Bay marine fish culture zone of Hong Kong. Bimonthly sediment samples were collected at six stations: two at the fish cages, two near the boundary of the fish culture area, and two reference sites further away from the culture area. Sediment physico-chemical characteristics in terms of silt/clay fraction, moisture content, total organic carbon (TOC), total Kjeldahl nitrogen (TKN) and total phosphorus (TP) were analyzed. The macrobenthos (>0.5 mm) present in the sediment were sorted, identified and enumerated. On average, TOC, TKN and TP levels at the fish cage stations were 82.8%, 128.5% and 1315.7% higher than those at the reference stations, respectively. As a result, the N:P molar ratio was greatly reduced from 8.75 at the reference stations to 1.83 at the fish cage stations. Univariate and multivariate analyses revealed that diversity of macrofauna was significantly reduced and community structure differed at the fish cage stations relative to the reference sites. The intermediary stations near the fish culture area showed a transitional state of disturbance. Faunal diversity was negatively correlated with nutrient level, reflecting the adverse impacts of nutrient enrichment derived from fish farming activities on the benthic assemblages. Whilst in subtropical Asia-Pacific trash fish is the major feed for fish culture resulting in a higher nutrient loading and nutrient ratio accumulated in the sediment beneath the fish rafts, the effects of nutrient enrichment on macrobenthic assemblages are comparable to that in temperate waters owing to relatively high sediment metabolism rate and smaller fish culture scale in Hong Kong.

Hydrodynamic Tracking of the Massive Spring 1998 Red Tide in Hong Kong

In subtropical coastal waters around Hong Kong, algal blooms and red tides have been frequently observed over the past two decades. In particular, in March—April 1998, a massive red tide invaded the northeastern and southern coastal waters of Hong Kong. The devastating red tide resulted in the worst fish kill in Hong Kong’s history, the most significant impacts being at the Lo Tik Wan and Sok Kwu Wan fish culture zones on Lamma Island. This work reports the first scientific investigation of the cause of this massive red tide. A calibrated three-dimensional (3D) hydrodynamic model for the Pearl River Estuary, \IDelft3D\N, is applied to study the advective transport of red tides. Based on the tidal boundary conditions and the measured wind data for a typical spring season, the 3D flow field is computed and extensive surface drogue tracking performed for releases in different parts of the coastal waters and for different tidal and wind conditions. The results show that a bloom initiated in Mirs Bay (Nan Au or Tap Mun) in the northeastern water would likely be transported to the southern coastal waters under the combined action of tidal current and wind. The computed bloom tracking patterns are generally supported by observations and are consistent with the temporal and spatial patterns of individual fish kill events in the 1998 red tide. We conclude that the major cause of the bloom being transported into the southern waters and East Lamma Channel (and causing the massive fish kill) is the generally strong wind in March—April 1998 and the change in wind direction in early April under almost diurnal tidal conditions. Further, it is most probable that the red tide originated in Mirs Bay rather than from outside Hong Kong. The findings provide a firm basis for environmental and fisheries management.

Zooplankton community structure of the sea surface microlayer near nuclear power plants and marine fish culture zones in Daya Bay

The authors' surveys in May–June 1999 (two cruises) at six sampling stations near nuclear power plants (NPP) and marine fish culture zones in Daya Bay, Guangdong, revealed species composition, densities and body-size of the sea surface microlayer (SM) zooplankton (>35 μm). Results showed that protozoans and copepod nauplii were the predominant components, accounting for 65.40% to 95.56% of total zooplankton in abundance. The size-frequency distributions showed that the frequency of micro-zooplankton (0.02–0.2 mm) reached 0.8235. The SM zooplankton community structure revealed in the present study was quite different from that revealed by investigations in the 1980s in Daya Bay. Difference of sampling method has important influence on the obtained zooplankton community structure. SM zooplankton consisted of micro- and mesozooplankton (0.2–2.0 mm), with micro-zooplankton being predominant. Some possible cause-effect relations between the zooplankton community structure and mariculture, nuclear power plants cooling systems and sampling method are discussed.

Water Quality Model for Mariculture Management

In marine fish farms located in subtropical eutrophic coastal waters, the dissolved oxygen (DO) level is often intimately related to algal growth dynamics. For effective mariculture management, a simple, tractable model that can elucidate the dependence of water quality on the important hydrographic, meteorological, and ecological parameters is required. A quasisteady water quality model is formulated herein to predict the long-term (seasonal) average concentrations of phytoplankton, organic nitrogen, dissolved inorganic nitrogen, and potential dissolved oxygen drop (PDOD) for fish farms located in weakly flushed tidal embayments. The fish culture zone is treated as a completely mixed nitrogen-limited system in which algal carbon is decomposed and nutrients regenerated continuously. The model assumes Monod-growth type relations for algal growth and nutrient uptake and incorporates algal settling and respiration, and tidal flushing. Using realistic estimates of light and temperature limiting factors and kinetic rate coefficients, model predictions of seasonally averaged water quality and qualitative features are well supported by field data.

Forecasting of Dissolved Oxygen in Marine Fish Culture Zone

Short‐term trends in dissolved oxygen in a weakly flushed marine fish culture zone in Hong Kong are studied by a real‐time water‐quality model with seven system variables: dissolved oxygen, chlorophyll‐a, organic nitrogen, ammonia, nitrate, zooplankton, and sediment algal carbon. The carbon‐chlorophyll ratio is computed as a function of a time‐variable saturating light intensity. Sediment oxygen demand is related to organic loads from the fish farm as well as algal biomass. Water‐quality predictions are compared with well‐documented field observations for both background and algal bloom conditions, over time scales on the order of a month. Model simulations demonstrate the importance of light intensity and photosynthetic production in dissolved oxygen variations, which are well predicted. The model also provides useful insights into the causes of severe oxygen depletion and phytoplankton dynamics in a eutrophic subtropical coastal marine ecosystem.

Dissolved Oxygen Variations in Marine Fish Culture Zone

Extensive field measurements aimed at studying short-term dissolved oxygen (DO) variability have been carried out in a marine fish culture zone in Hong Kong. A dynamic oxygen budget model is developed to analyze the diurnal water-quality data from a series of 26-hr surveys. The model formulation includes phytoplankton dynamics and photosynthetic production, sediment oxygen demand (SOD), fish respiration, carbonaceous and nitrogenous oxidation, and atmospheric oxygen transfer. The model is used to determine the components of oxygen consumption and production—in particular, the SOD and carbon-to-chlorophyll (CCHL) ratio. It is found that SOD and algal respiration are major oxygen consumption items, and the CCHL ratio is much higher than reported in previous mathematical model studies. Simulations of the diurnal DO variation are compared with data over a wide range of meteorological and ecological conditions. The model interpretation provides a basis for the formulation of a water-quality model for short-term DO forecasts.