## 1 Introduction  
The demand for new protein sources and the better utilization of existing resources in the food industry is increasing globally, and fish and fish products will play a crucial role in food security [1]. Fish is an excellent source of proteins and lipids, providing a balanced amino acid (AA) composition and polyunsaturated fatty acids (PUFAs), as well as minerals and vitamins. Increased demand has led to overfishing and the depletion of commonly used fish stocks, while many other fish resources are still under-utilized. For instance, in Finland, Baltic herring (_Clupea harengus membras_) (B. herring) is commercially the most significant fish, with its annual catch exceeding 100,00 tons, but most of the catch is used as feed [2]. B. herring is a small, dark-muscled fish, usually 15-20 cm in length, and has a relatively high lipid content, despite the average lipid content having decreased from 7.7% in 1988 to 4.3% in 2017 [3]. Due to the seasonal variation in fishing, fresh B. herring is not available all-year-round. Roach (_Rutilus rutilus_) is a cyprinid fish that has a low economic value and is utilized as food even to a lesser extent than B. herring. Increasing fishing of roach also has the potential to decrease eutrophication, since removing roach from the water systems directly removes nutrients, and as a protein-rich food roach contributes less to global warming compared with several commonly used protein sources, such as milk, poultry, beef, pork, eggs, and meat substitutes [4]. A potential way to increase the food use of low-value fishes is fractionation, which allows the separation, enrichment, and valorization of their proteins and lipids. Small-sized whole fish, such as a large proportion of B. herring that are too small to be filled industrially and that currently mostly end up as feed, could also be valorized by fractionation. Enzymatic hydrolysis has been commonly applied to recover peptides from several plant- and animal-based raw materials and by-products from meat and fish processing. Some of the resulting peptides have bioactivities, such as antioxidative and ACE I- inhibiting effects [5; 6]. Several enzymes, such as pepsin, trypsin, \(\alpha\)-chymotrypsin, pan-creatin, papain, and commercial enzyme products such as Alcalase, Flavourzyme, Neustrase, and Protamex have been applied [6]. Treated fish and by-products include, for instance, salmon (_Salmo salar_) frames [7], Pacific whiting (_Merlucius products_) [8] and its by-products [9], yellow stripe trevally (_Selaroides leptolepis_) [10], brownstripe red snapper (_Lutjanus vitita_) [11], turbot (_Scophthalmus maximus_) by-products [12], whitemouth croaker (_Micropogonias furunieri_) [13], and banded croaker (Paralonchurus brasiliensis) [13]. Alkaline or acidic extraction followed by isoelectric precipitation, also known as the pH shift, has been applied to fractionate several fishes and their by-products, such as herring (_Clupea harengus_) [14; 15], bigeye snapper (_Priacanthus tayenus_) head [16], by-products of rainbow trout (_Oncorhynchus mykiss_) [17], by-products of silver carp (_Hypophthalmichthys molitrix_) [18], and by-products of salmon (_Salmo salar_), cod (_Gadus morhua_), and herring [19]. In the process, proteins are first solubilized using an acid or base to increase their positive or negative net charge, inducing protein-protein electrostatic repulsion, and increasing protein-water interactions. After solubilization, pH is adjusted to the isoelectric point of the protein, where proteins have a net charge close to zero, causing precipitation. The pH shift process has the advantage that the proteins are not significantly cleaved or do not have to be subjected to a heat treatment and hence retain certain functional properties, such as gelation [15]. The quality of the resulting protein isolate or hydrolysate determines its applicability. The composition of amino acids, as well as the digestibility and bioavailability of the protein, is essential in terms of nutritional value [20].