Proximate Composition of Fishery By-Catch from a Small-Scale Anchovy Processing Facility in Papar, Sabah, Malaysia

2.4 Derived proximate composition indices of fishery by-catch meal

The derived proximate composition indices of FBCM, including the protein-to-ash ratio, lipid-to-protein ratio, lipid-to-ash ratio, approximate organic fraction, variability range, and coefficient of variation (CV), were calculated from the monthly proximate composition data using the following formulas:

1. Protein-to-ash ratio = Crude protein (%) / Ash (%)
2. Lipid-to-protein ratio = Crude lipid (%) / Crude protein (%)
3. Lipid-to-ash ratio = Crude lipid (%) / Ash (%)
4. Approximate organic fraction (%) = 100 - Moisture (%) - Ash (%)
5. Variability range = Maximum value - Minimum value
6. CV (%) = [Standard deviation / Mean] × 100

The composition of FBC varied among the monthly sampling batches, with no consistent temporal trend observed throughout the six-month sampling period. This variation may have been influenced by a combination of ecological, environmental, and fishing-related factors. In Sabah, Malaysia, anchovy fishing is traditionally conducted using ‘bagang’, a lift-net fishing method operated at night from bamboo or wooden platforms installed in nearshore waters, where artificial light is used to attract fish. This fishing practice may influence by-catch composition by favoring the capture of species that are attracted to surface light sources and forage in the upper water column. For instance, several fish groups, including mackerels, ponyfishes, terapons, and sardines, were consistently recorded throughout all sampling months. Their frequent occurrence may be attributed to their feeding behavior, as these species commonly aggregate and feed near the water surface, making them more susceptible to capture by bagang operations [10-12]. In contrast, benthic fish groups such as seamoths, saddle grunts, and sculpins were recorded only occasionally, likely because their bottom-associated habitats reduce their probability of encountering surface-oriented lift nets [13-15]. Nevertheless, this study was conducted at a single anchovy landing site in Sabah, and the observed by-catch composition may not represent the diversity and abundance patterns across other landing locations. Differences in local environmental conditions, habitat characteristics, and fishing practices among sites may contribute to variations in by-catch species composition. Therefore, further investigations involving multiple anchovy landing sites and longer sampling periods are recommended to better understand the spatial and temporal dynamics of FBC composition in Sabah.

Based on the total by-catch mass, fish accounted for an average of 94.16 ± 1.97% of the by-catch across the six-month sampling period, whereas squid, shrimp, and crab collectively contributed less than 1%. The remaining proportion comprised miscellaneous materials, which were excluded from FBCM production. Owing to their low proportions, the squid, shrimp, and crab fractions were also excluded, and only the fish fraction was processed into meal. The exclusion of crustaceans may also have helped avoid an additional source of mineral matter, as their highly calcified exoskeletons are rich in minerals [16]. Nevertheless, the resulting FBCM still exhibited a relatively high ash content, ranging from 31.58% to 37.86%. This relatively high ash content may limit the potential applications of FBCM, as a substantial proportion of ash reduces the relative contribution of organic nutrients in the meal. Similarly, although meat and bone meal contains approximately 50% crude protein, its use as a fishmeal substitute has been limited largely because of its high ash content (approximately 29%) [17]. Excessive dietary ash has also been reported to reduce nutrient and protein digestibility in fish, thereby impairing feed utilization and growth performance [18, 19]. Therefore, pre-treatment approaches to reduce the ash content of FBCM should be explored to improve its nutritional value and potential application as a feed ingredient.

The relatively high ash content observed in the present study may be associated with the processing of whole fish, including their mineralized skeletal tissues. Although visible foreign materials, such as coral fragments, stones, seashells, and other debris, were carefully removed during sorting, acid-insoluble ash was not determined. Therefore, the relative contribution of fish-derived mineral matter and any residual inorganic materials to the total ash content cannot be confirmed. The 500 µm sieving step used in the present study was primarily intended to standardize meal particle size rather than to remove inorganic materials; consequently, fine mineral particles derived from fish bones may have remained in the final FBCM. Further studies should determine acid-insoluble ash and quantify mineral elements, particularly calcium, phosphorus, and sodium, to better characterize the mineral composition and potential sources of the high ash content. Appropriate pre-treatment strategies to reduce excessive mineral matter could also be investigated if subsequent studies demonstrate that the ash level limits particular applications of FBCM.

The crude protein and lipid contents of FBCM varied among the monthly sampling batches, ranging from 55.49% to 61.52% and from 3.79% to 5.26%, respectively. The observed variation may partly reflect differences in the composition of the by-catch. However, fish-group occurrence in the present study was recorded only as presence or absence rather than relative biomass; therefore, the contribution of individual fish groups to the observed nutritional variation cannot be directly established. Other biological and processing-related factors, including fish body size, viscera proportion, drying conditions, and storage duration, may also have contributed to the variation. Such variability in raw material characteristics is an important consideration in the utilization of FBC and processing by-products [20, 21]. Future studies could incorporate quantitative measurements of fish-group biomass together with relevant biological and processing parameters to better elucidate the factors contributing to temporal changes in FBCM composition. Sorting the by-catch into different fish groups before processing could also be investigated as a potential approach for improving the consistency of the resulting meal [6].

The derived proximate composition indices further illustrated differences in the relative composition of FBCM among sampling months. An apparent inverse pattern was observed between crude protein and ash contents, with higher ash contents generally coinciding with lower crude protein contents. This pattern was reflected in the protein-to-ash ratio, which was highest in April (1.95), when crude protein was highest and ash was lowest, and lowest in August (1.47), when crude protein was lowest and ash was highest. Similarly, the approximate organic fraction, expressed as the non-ash fraction, was highest in April (68.42%) and lowest in August (62.14%). These patterns may partly reflect differences in the relative contribution of protein-rich soft tissues and mineralized skeletal tissues among the monthly by-catch samples. However, because the relative biomass of individual fish groups and the contribution of bone-derived minerals were not directly determined, these relationships should be interpreted cautiously and do not establish a causal relationship between ash and protein contents.

In comparison, the lipid-to-protein and lipid-to-ash ratios showed relatively limited variation, ranging from 0.06 to 0.09 and from 0.11 to 0.14, respectively. The coefficients of variation indicated that crude protein was relatively consistent among the monthly sampling batches (CV = 4.26%), whereas crude lipid (CV = 13.08%) and moisture (CV = 10.71%) exhibited greater relative variability; ash showed intermediate variability (CV = 8.78%). These descriptive indices indicate that the magnitude of temporal variation differed among the proximate components. Nevertheless, proximate composition alone does not provide a complete assessment of the nutritional characteristics of FBCM. Given the observed monthly variation, temporal differences in other nutritional attributes cannot be excluded. Further studies should therefore evaluate essential amino acid and fatty acid profiles, mineral and vitamin composition, digestibility, and other relevant nutritional characteristics across sampling periods to better establish the nutritional consistency and potential applications of FBCM.

In addition to nutritional considerations, the occurrence of pufferfishes among the recorded fish groups warrants attention from a feed-safety perspective. Pufferfishes were not specifically removed before processing in the present study, and all fish material was ground together to produce FBCM. Consequently, the potential presence of naturally occurring toxins or other species-specific hazards in the resulting meal cannot be excluded. The present study also did not evaluate potential contaminants or safety indicators, such as heavy metals, natural toxins, or microbiological quality. Future processing strategies should therefore incorporate species-level screening and selective removal of potentially toxic fish before meal production, followed by appropriate safety assessments before the material is considered for feed applications.

A further limitation concerns the absence of mass measurements at the different processing stages. Although 30 kg of sun-dried FBC was procured for each monthly sampling batch, the masses of the separated fish fraction, material after oven drying, and final FBCM were not recorded. Consequently, material recovery and processing yield could not be determined. Sorting, additional drying, grinding, and sieving may result in material losses at different stages of processing. Future studies should therefore record the mass at each processing stage to establish the material balance, recovery rate, and processing efficiency of FBCM production.

The authors would like to thank the Malaysian Ministry of Higher Education for providing financial support through the Higher Institution Centre of Excellence (HICoE) Research Grant Scheme (approval letter ref. no. JPT(BKPI)1000/016/018/35(2); grant codes HIC2403 and HIC2405).